Rtb is much better than steam workshop, much like nexus mod manager is better and unlike the nexus, rtb is only slightly more complicated.
Steam Workshop actually works well for Garry's Mod, you can enable/disable mods, you can install tools, ragdolls, entities, maps, props, weapons and vehicles, they are ready to be used immediately, certainly a lot quicker than downloading from a site like garrysmod.org, extracting the archive file and putting folders/files in the correct area(s) of the garrysmod/garrysmod directory
Garry's Mod's Toybox and Garry's Mod Beta's workshop are like RTB except you can't see more than one picture of the mod, you can still rate, comment, enable/disable, and delete