Are teaching on gratitude and "duty" nessesary?

Should Gratitude be on a par with Loving Kindness?

“These two people are hard to find in the world. Which two? The one who is first to do a kindness, and the one who is grateful and thankful for a kindness done.” — AN 2:118

Here the Buddha advises us to treasure people with Kindness and gratitude when we find them, and for us to become such rare persons ourselves.

Kindness and gratitude are virtues that needs to be cultivated together to be genuine and heartfelt. Gratitude teaches kindness as you’ve benefitted from another person’s actions, and as you trust the motives behind those actions. Only if you’ve been kind to another person will you accept the idea that others can be kind to you. At the same time, if you’ve been kind to another person, you know the effort involved. You sense that the other person had to go out of his or her way to provide that benefit. So, when you’re on the receiving end of a sacrifice, you realize you’ve incurred a debt, an obligation to repay the other person’s trust. Therefore, the Buddha always discusses gratitude as a response to kindness, and it is more than an appreciation in general.

Are teaching on gratitude and "duty" nessesary for the path development?

AN 2:31 - Kataññu Suttas: Gratitude tells us that one should treat one’s father and mother with great respect and never be remiss in one’s filial duty.

“I tell you, monks, there are two people who are not easy to repay. Which two? Your mother & father. Even if you were to carry your mother on one shoulder & your father on the other shoulder for 100 years, and were to look after them by anointing, massaging, bathing, & rubbing their limbs, and they were to defecate & urinate right there [on your shoulders], you would not in that way pay or repay your parents. If you were to establish your mother & father in absolute sovereignty over this great earth, abounding in the seven treasures, you would not in that way pay or repay your parents. Why is that? Mother & father do much for their children. They care for them, they nourish them, they introduce them to this world.

“But anyone who rouses his unbelieving mother & father, settles & establishes them in conviction; rouses his unvirtuous mother & father, settles & establishes them in virtue; rouses his stingy mother & father, settles & establishes them in generosity; rouses his foolish mother & father, settles & establishes them in discernment: To this extent one pays & repays one’s mother & father.“— AN 2:32

Is it possible to gain path and fruits for an ungrateful person, a person denying his/her duties?

Acts of kindness and gratitude teach you valuable lessons about kindness and empathy in the process. The debt you owe your parents for giving birth to you and enabling you to live is immense. The only true way to repay your parents is to strengthen them in four qualities: conviction, virtue, generosity, and discernment. To do so, you must develop these qualities in yourself, as well as learning how to employ great tact in being an example to your parents. These four qualities are also those of an admirable friend, and thus you become that admirable friend to yourself. Thus it is this very same admirable friend within you that will help you to to gain path and fruits.

Could it be that the seldom appearing of gratitude is direct connected with the seldom appearing of people reaching path and fruit?

As I have said before, these four qualities (conviction, virtue, generosity, and Discernment) are what makes you become that admirable friend to yourself. Thus, it is this very same admirable friend within you that will help you to gain path and fruits. You become a person of integrity, who has learned from gratitude how to be harmless in all your dealings and to give help with an empathetic heart: respectfully, in a timely way, and with the sense that something good will come of it (MN 110 - Cūḷapuṇṇama Sutta; AN 5:148 - A Person of Integrity's Gifts Sappurisadāna Sutta); MN 103 - Kinti Sutta.

Gratitude also gives practice in developing qualities needed in meditation. The practice of concentration centers on the power of perception. Training in gratitude shows how powerful perception can be, for it requires developing a particular set of perceptions about life and the world. If you perceive that the goodness in life is the result of cooperation, then the give and take of kindness and gratitude become a pleasant exchange. Gratitude requires mindfulness. In Pali, the word for gratitude— kataññu—literally means to have a sense of what was done.

In SN 48:10, the Buddha defines mindfulness as “remembering & able to call to mind even things that were done & said long ago.” Our parents’ instructions to us when we were children—to remember the kindnesses of others—are among our first lessons in mindfulness. As we develop our sense of gratitude, we get practice in strengthening this quality of mind. When these qualities are not found in the practitioners , the path and fruit too are seldom reached by them.

What in particular are teaching on gratitude mentioned in the suttas?

Buddha cited gratitude as the quality defining what it means to be civilized. AN 2:31 - Kataññu Sutta. -Gratitude

In SN 15:14-19 - Sukhita Sutta Buddha said:

“A being who has not been your mother at one time in the past is not easy to find…. A being who has not been your father … your brother…. your sister…. your son…. your daughter at one time in the past is not easy to find. Why is that? From an inconceivable beginning comes transmigration.” “Long have you thus experienced stress, experienced pain, experienced loss, swelling the cemeteries—enough to become disenchanted with all fabricated things, enough to become dispassionate, enough to be released.”

The above sutta shows that not all the lessons taught by gratitude and empathy are of a heartwarming sort. Instead, they give rise to a sense of samvega. You know that not all your past intentions have been skillful, and yet these are the things that will shape the conditions of your life now and into the future. This understanding make you want to find a way out even of the network of kindness and gratitude that sustains whatever goodness there is in the world.

To identify yourself as a being means having to find food—both physical and mental—to keep that identity going. Therefore, when you’re a being, you need to depend on a network of kindness, gratitude, and sacrifice. That is why you are advised to think thus:

“We will undertake & practice those qualities that make one a contemplative… so that the services of those whose robes, alms-food, lodging, and medicinal requisites we use will bring them great fruit & great reward.” — MN 39 - Maha-Assapura Sutta: The Greater Discourse at Assapura

At the same time, the example of your behavior and freedom of mind is a gift to others, in that it shows how they, too, can free themselves from their debts. This is why the Buddha said that only those who have attained full awakening eat the alms food of the country without incurring debt. They’ve even paid off their debt to the Buddha for having taught the way to release.

The only homage he requested was that people practice the Dhamma in line with the Dhamma — i.e., to develop the disenchantment and dispassion that lead to release (DN 16 – Maha Parinibbana Sutta; SN 22:39-42 Anudhamma Sutta. In Accordance with the Dhamma) — so that the world will not be empty of awakened people. In this way, attaining full release is not a selfish act; instead, it’s the highest expression of kindness and gratitude.

Teaching on gratitude & “duty” is not necessary for Nibbana. MN 117 states gratitude & duty is a defiled (asava) right view with attachment (upadhi) siding with merit. In MN 56, Upali's stream-entry resulted in independence from the teacher. In the Vakkali Sutta, Vakkali was rebuked for wrong gratitude. Dhammapada 294 states to "kill the mother & father to attain Nibbana". In MN 86, the arahant Angulimala, ate his alms food without debt (obligation). Gratitude is a moral teaching for worldlings (puthujjana) but is unrelated to the goal of Buddhism, namely, Nibbana. Dhp 43 says:

Na taṃ mātā pitā kayirā, aññe vāpi ca ñātakā; sammāpaṇihitaṃ cittaṃ, seyyaso naṃ tato kare

Neither mother, father, nor any other relative can do greater good than the well-directed mind.