Did Gautama Buddha believe in God?

Gauthama buddha wanted to find the Ultimate Truth. He has undederstood it. A person who understand it born in for periods of kalpa(For billions of years)

Gods are also under that truth. No one can escape it. The prime Anicca cannot be bypassed even for gods. They also ends-up their powers as gods one day.

Certain Seela or customs protected by Bhikkhus can only be protected by humans. So it is told that gods worship for those people who protect advanced customs. So gods worships and believe in Triple Gems(Buddha, Dhamma, Sanga).

Buddha accepted supernatural powers, and supernatural realms. But he never believed that they are the best because supernatural powers and realms still arise and vanish. So they still are suffering.

As mentioned in another thread by @catpnosis, in MN 100: Sangarava Sutta:

“kiṃ nu kho, bho gotama, atthi devā”ti?

"How is it then, good Gotama; are there divine beings?"

“ṭhānaso metaṃ, bhāradvāja, viditaṃ yadidaṃ — adhidevā”ti.

"With reason it is known by me, Bhāradvāja, that there are higher divinities."

The question is, what do you mean by belief? If you mean believe to have existed certain beings who were believed to be divine, omnipotent, eternal, etc., then yes, it is quite clear the Buddha not only believed in such beings but, according to the texts, had direct experience interacting with them.

If you mean actually believing that such beings were omnipotent, omniscient, eternal, etc., then no he clearly had no such belief in regards to said beings. Reading the Brahmajala Sutta (DN 1) should be enough to dispel that idea. He does give a nice explanation as to where such belief comes from:

39. "There comes a time, bhikkhus, when after the lapse of a long period this world contracts (disintegrates). While the world is contracting, beings for the most part are reborn in the Ābhassara Brahma-world.[7] There they dwell, mind-made, feeding on rapture, self-luminous, moving through the air, abiding in glory. And they continue thus for a long, long period of time.

40. "But sooner or later, bhikkhus, after the lapse of a long period, there comes a time when this world begins to expand once again. While the world is expanding, an empty palace of Brahmā appears. Then a certain being, due to the exhaustion of his life-span or the exhaustion of his merit, passes away from the Ābhassara plane and re-arises in the empty palace of Brahmā. There he dwells, mind made, feeding on rapture, self-luminous, moving through the air, abiding in glory. And he continues thus for a long, long period of time.

41. "Then, as a result of dwelling there all alone for so long a time, there arises in him dissatisfaction and agitation, (and he yearns): 'Oh, that other beings might come to this place!' Just at that moment, due to the exhaustion of their life-span or the exhaustion of their merit, certain other beings pass away from the Ābhassara plane and re-arise in the palace of Brahmā, in companionship with him. There they dwell, mind-made, feeding on rapture, self-luminous, moving through the air, abiding in glory. And they continue thus for a long, long period of time.

42. "Thereupon the being who re-arose there first thinks to himself: 'I am Brahmā, the Great Brahmā, the Vanquisher, the Unvanquished, the Universal Seer, the Wielder of Power, the Lord, the Maker and Creator, the Supreme Being, the Ordainer, the Almighty, the Father of all that are and are to be. And these beings have been created by me. What is the reason? Because first I made the wish: "Oh, that other beings might come to this place!" And after I made this resolution, now these beings have come.'

"And the beings who re-arose there after him also think: 'This must be Brahmā, the Great Brahmā, the Vanquisher, the Unvanquished, the Universal Seer, the Wielder of Power, the Lord, the Maker and Creator, the Supreme Being, the Ordainer, the Almighty, the Father of all that are and are to be. And we have been created by him. What is the reason? Because we see that he was here first, and we appeared here after him.'

43. "Herein, bhikkhus, the being who re-arose there first possesses longer life, greater beauty, and greater authority than the beings who re-arose there after him.

44. "Now, bhikkhus, this comes to pass, that a certain being, after passing away from that plane, takes rebirth in this world. Having come to this world, he goes forth from home to homelessness. When he has gone forth, by means of ardor, endeavor, application, diligence, and right reflection, he attains to such a degree of mental concentration that with his mind thus concentrated he recollects his immediately preceding life, but none previous to that. He speaks thus: 'We were created by him, by Brahmā, the Great Brahmā, the Vanquisher, the Unvanquished, the Universal Seer, the Wielder of Power, the Lord, the Maker and Creator, the Supreme Being, the Ordainer, the Almighty, the Father of all that are and are to be. He is permanent, stable, eternal, not subject to change, and he will remain the same just like eternity itself. But we, who have been created by him and have come to this world, are impermanent, unstable, short-lived, doomed to perish.'

http://www.accesstoinsight.org/tipitaka/dn/dn.01.0.bodh.html

The Jataka commentary also has a nice passage about god and the godly:

These greedy liars propagate deceit,
And fools believe the fictions they repeat;

He who has eyes can see the sickening sight;
Why does not God set his creatures right?

If his wide power no limits can restrain,
Why is his hand so rarely spread to bless?

Why are his creatures all condemned to pain?
Why does he not to all give happiness?

Why do fraud, lies, and ignorance prevail?
Why triumphs falsehood,—truth and justice fail?

I count your God one th’ injust among,
Who made a world in which to shelter wrong.

Jāt. 543