Did the Gentiles believe God before the gospel came to them?

No, the Gentiles for thousands of years did not believe God. They may have believed in some God, but they did not believe God, meaning they did not take Him at His Word. Believing God is faith; believing in God does not necessarily involve faith.

Les explains the difference between believing in God and believing God. He notes that most people, especially in America, will admit they believe in God, but that does not require faith. Believing God, however, means taking Him at His Word, and that is faith. The Gentiles for thousands of years did not believe God; they may have believed in some God, but they did not believe God. This distinction opened people's eyes and made them rethink what they had learned.

Romans 11:30

For as ye in times past have not believed God, yet have now obtained mercy through their unbelief:

What did the Gentiles do for thousands of years regarding belief in God?

For thousands of years, the Gentiles did not believe God. They may have believed in some god, but they did not believe God. This distinction separates believing in God from believing God, which is faith—accepting what He says as true.