Yes, this has been said hundreds of times, but for the sake of not putting you through a search, ill just tell you.

With the right amount of dedication and know-how, almost anything can be done with 3DGS. Pretty much, if you think it, you can do it. So, that being said, you are going to have to do A LOT of learning. A doom1 style game can be harder to make than you think.

Raddaddy