Garry's Mod (as a game development "platform") essentially boils down to a Lua interpreter bolted to the Source Engine, with access to assets from Valve's games. If you want to make a new game(mode) in Garry's Mod, you still have to write code, design levels and possibly create models, textures and sounds in separate somewhat hard to use programs.

That's very different from Dreams, where you can create everything within the same program with a unified set of pretty accessible tools and a very fast iteration cycle.