Is Garry's Mod getting old?

Depends which modern game are you talking about. It mostly lives because of community mods and most modded games are old ones. I can't think of any modern game which has more freedom of mods. some even use it as a tool to create cool pictures like ( my favorite artist is Olmate Ubafest). My favorite part about it was that you could decorate rooms/houses with props and ragdolls and animate them with addons so it would be more lively. honestly it's way easier than Blender.