The problem with gmod is addons, to map in hammer you have to mount the addon games. I've previously had a look in to it and its relatively easy to do with the use of a program which will launch hammer with the correct version and addons mounted so long as you aren't incapable of basic tasks.
Suurce multi tool is just an alternitive method of launching source sdk, all it really does is simplify the way you launch and mount games to hammer.
https://facepunch.com/showthread.php?t=1327883
there should be a few videos about the tool and how to mount it for gmod, but normally you just want to mount the normal models people will have, such as; hl2,gmod,css and hl2eps1/eps2
mounting half-life episode 1 and 2 isn't always a great idea cause the kiddies on gmod can only afford gmod and maybe hl2 at a push. but obviously you can mount this as you see fit for what ever you are doing.
edit: also the ESC have good videos for making map which can be used in theory for any game/map ideas you have so long as you do it in hammer.
https://www.youtube.com/watch?v=wSEd-8y ... odNZTe3ndX