Buddha is a God or different God's level misconception in Buddhism System

You might call him a "philosopher", and a great one. There are debates about whether Buddhism is a religion or a philosophy -- if it's a philosophy then he's a philosopher. SFAIK most mainstream Christians are OK with studying secular philosophy -- like Socrates, and who knows who else.

He was also a "teacher". And arguably "a holy man", both according to the form of the time (i.e. a homeless monk) and IMO by contemporary standards too according to the ethics he taught and practised.

He's earlier than Jesus, passing away approximately 400 BCE. I believe that Muslims for example are permissive of other people who were never Muslims, but, less tolerant people who were Muslims but who then left the faith (i.e. apostates).

You could say that Buddhism doesn't claim to be the Word of God in the Christian sense -- it's a study of human life, culture, wisdom.

You don't "serve" the Buddha -- you learn Buddhist doctrine and practice it because it's common-sense (it's practical, beneficial, well-explained, and so on).

https://erlc.com/resource-library/articles/4-ways-christians-can-respond-to-a-satanic-statue

So that's written by a Southern Baptist, eh? Oh well. Yeah apparently they're not keen on statues -- they are low church. Do you want to say which sect/denomination your girlfriend is, in case that helps to know?

Anyway I guess you're talking about this sentence in it:

“What the pagans sacrifice is to demons.” Every golden Buddha in your city is a statue to Satan, as well as every Hindu idol.

Do you "sacrifice" to the statue, worship it? People do, or seem to, in temples and with Buddhist shrines. I think the early Buddhist texts (the suttas) don't mention that kind of practice. There were lots of people being polite (reverential) to the Buddha and other monks socially i.e. when they were alive and having conversations. Maybe "sacrifices" in Buddhism tend to be not to statues, but instead or also donations (e.g. of alms food and other necessities) to the Sangha.

Possibly even Christians could approve of that: it's "Christian Charity" and the "Great Commandment" (love thy neighbour), which (see also the "Good Samaritan") Christ teaches one should love to give to any human, any neighbour, who needs it -- I'm pretty sure Jesus praised that kind of behaviour...

Still beware of trying to teach them their own theology. Jesus got into a lot of trouble himself, by doing that. Try to be more low-key about it, for example, "This isn't 'sacrificing' -- it's being 'charitable', and learning not be avaricious...".

Also I'm not sure that Buddhism is "an opponent religion". Read SN 7.2 again -- the Buddha doesn't get involved with opposing the brahman. Similarly I don't know that Buddhism has to be "opposed to" Christianity. In fact Buddhism (unlike some Christians) tends not to proselytise, isn't that so? Without there being opposition or competition, perhaps there should be no enmity.

I fear a lot of arguments come from "black and white thinking", "you're either with us or against us" -- which I don't think is sensible but I'm not sure how to solve it (ruben2020's answer already referenced the Dhammapada). Buddhism might teach a middle way -- avoiding the mistakes associated with extremism -- and warns that people get into sectarian disputes because they hold to specific views and so on (see the parable of the blind men and the elephant -- Ud 6.4)

See also:

• To debate or not to debate? and When to debate? and How to debate?
• How are Buddhists supposed to spread Buddhism? (where the accepted answer argues that the doctrine is spread, but not via "in-your-face Christian style evangelism", another suggests it's taught "by example")

Well, i answered Mr. Gautama is not God, a human being like us and just a superior teacher?

Maybe beware of using the word "superior". That's in your title as well -- "above God".

Having read this answer I associate comparisons with conceit (and "superior" is a comparison).

And "conceit" is associated with disrepect etc. -- read about Māna.

(I think "pride" is also the "original sin" according to christianity, even the reason for Satan's own fall from grace).

It may be safer (less confrontational) to say that he's "a teacher" not "a superior teacher".

Another thing that might be worth mentioning is that Christians seem to me to put a lot of emphasis on the person-hood of Christ (and the Trinity).

That's almost the opposite of what Buddhism teaches, e.g. that person-hood is mere convention. I think the Buddha also taught in various ways that what's important is the Dhamma, moreso than the Buddha -- see Reference request for "the Buddha takes the Dhamma as his superior" -- see also this from DN 16, nearly the Buddha's last words:

Now, Ānanda, some of you might think: ‘The teacher’s dispensation has passed. Now we have no Teacher.’ But you should not see it like this. The teaching and training that I have taught and pointed out for you shall be your Teacher after my passing.

IMO "Who is superior, is it Christ or Buddha?" is not a sensible question. It's better to focus on "What is the Dhamma?" and "What is good about the Dhamma?" -- or "What qualities, practices does Buddhism consider virtuous, skilful?" -- I think that's more beneficial, not a "my Dad is greater than yours" kind of argument.

There is no single god-almighty-creator in buddhism. So neither is Shakyamuni Buddha, period.

You can portray Gautama Buddha as an ordinary man who lived 2500 years ago, who found the way to end suffering and attain true happiness.

He discovered that suffering is mostly a mental condition. Thus, Gautama Buddha became history's most illustrious psychologist.

To support this claim, you can quote the first 6 verses of the Dhammapada. Part of these verses sound similar to Jesus' turning the other cheek, thus making Gautama Buddha appear compatible with Jesus in just this way.

Also, statues of Gautama Buddha are just for inspirational purposes, similar to statues of Abraham Lincoln, Martin Luther King and Mahatma Gandhi.