Gigas Created the Boulder, the Popsicle, and the lump of steelRegigigas? Yes. Mega Regigigas? No. You'll need the 3 other regis first, and that'll probably require trading though.
It's not about choosing, it's a fact that the anime (even specials based off the games) and the games have no affect on each other. What it does prove is that Pokemon is ok with the idea that MEvolution can exist in other regions. But nobody was really suggesting that you wouldn't be able to use it at all in ORAS.