Gmod alternatives?

It really depends on what it is about Gmod you want an alternative to. If you're looking for somewhere to make RP servers, GTA (SA or 5) is a viable option on the varied 3rd party hosting programs. But if you're looking for the ultra-customizability that GMod offers, there isn't really many alternatives. That's probably why despite being as old as it is, GMod still makes sales and has players.