He was/is called many thing, including his birth name. He also went by:
(the) Buddha. Buddha means "Enlightened One."
The Arahant
The Tathagata (Means One who has thus gone and one who has thus come)
The Perfectly Enlightened One
Gautama Buddha
Buddha Shakyamuni (Means Sage of the Shakyas)
Siddhartha Gautama is Buddha. He is also known as "The Enlightened One".
Siddartha Gautama is the real name of Buddha. He was commonly known as Buddha because he created the belief of Buddhism. But his real name is Siddartha Gautama.
Siddhartha Gautama
The last name of the Buddha before he became enlightened was Siddhartha. It was after his enlightenment that he was known as Gautama Buddha.
Buddha was originally referred to as Siddhartha Gautama or by Gautama Buddha.
Buddha Shakyamuni (Siddhartha Gautama)
~ gautama siddhartha
Siddhartha Gautama is referenced by a number of different titles, but mostly as Buddha. Sometimes he is referred to as Sakyamuni Buddha where Sakyamuni (Sage of the Sakyas) is a refernce to the tribe he was born into (Sakya) or at times he is called Gautama Buddha, as you indicated, where Gautama is his family name. Both of these are used when referring to the actual historical person of Siddhartha Gautama as opposed to another enlighten person who has attained Buddhahood (who would be called a Buddha also).
The Buddha was originally named Siddhartha Gautama, who was born in Kapilvastu, a small village in Nepal. After his enlightenment he was called by the honorific "Buddha" or the enlightened/awakened one.
Siddhartha Gautama who reached enlightenment and became know as The Buddha.
There is no Buddhist god, neither is Buddha god - he's just a man. Buddha was quite clear that the presence or absence of gods does not make any difference to our progress towards enlightenment.
Buddha was originally called Siddhartha Gautama.