The God of the gaps idea is that since there could be no possible natural explanation, God must have done it. God-as-first-cause is a different argument, because God is the first cause whatever it is, even a natural one.

I will reserve judgement, but I don't expect many people accept whatever explanation scientists eventually produce for the beginning of existence. What I expect is that when scientists explain first cause, the "God-as-first-cause" argument will fade away, and, say, "God-as-abiogenesis" will become more popular. Supporters of that will attempt to distinguish it from a typical god-of-the-gaps argument by claiming that whatever process caused life to spring into existence is God.