Siddhartha Gautama, or Buddha, was created by God, yes.
Everyone who is not God was created by God (Genesis 1:27; Ecclesiastes 7:29; Isaiah 46:9), and God cannot lie (Titus 1:2; Hebrews 6:18), and is never wrong (Colossians 2:3; John 21:17; 1 John 3:20). If Buddha claimed there is no ultimate creator God, then either he was wrong or he was lying, thereby identifying himself as not-that-god, since God cannot lie and is never wrong. Since Buddha was not-that-god, he was, like everything that is not-that-god, created by the only creator God (John 1:3; Colossians 1:16).
All humans, other than Jesus the Christ (Hebrews 4:15), have sinned, or disobeyed the God who created them (Romans 3:23, 5:12). This includes Siddhartha Gautama. As a result, Gautama, like everyone else, deserves both temporal death and the second, everlasting death (Romans 6:23; Revelation 21:8).
If we do not have the faith of Abraham in the God who both created us and can save us from the penalty of sin (Romans 4:3-8), we have no guarantee of forgiveness. Rather, the Bible says that all unbelievers will be cast into the second death (Revelation 21:8), and that all are without excuse (Romans 1:20-21; Psalm 14:1).
I am far from an expert on the life and faith of Buddha. Whether or not Siddhartha Gautama himself is in hell, experiencing the second death, I myself cannot say with confidence. It would appear from what the Bible says and from the assumptions in the question itself, namely that Gautama denied the existence of his own creator God and the only savior God that would be offered to him, that yes, he is currently attempting to pay the penalty for his own sins where the worm does not die and the fire is not quenched (Isaiah 66:24; Mark 9:43-48).
Regardless of what state Siddhartha Gautama is currently in, you the reader can be right with God in spite of your own disobedience against Him. By asking for God's forgiveness, and placing your life in the hands of Jesus, by trusting His sacrifice on your behalf, if you have not already done so, you can be forgiven (Romans 5:8; Acts 2:21; Romans 10:9-10).
Whether or not it is too late for Buddha, it is not too late for us! (2 Corinthians 6:2)