So you hate yourself and want to open Hammer! Congrats! To make maps on GMOD or well other applications. Click on GMOD in your library! Click on the settings icon in the gmod's library page. Then go to manage click on browse local files. Then click on bin and you should scroll down to find hammer.exe. If my instructions are not clear enough which they probably aren't. I will leave the file path for you to copy and paste!

System file path: C:\Program Files (x86)\Steam\steamapps\common\GarrysMod
This should take care of that! Now please make maps step by step and WATCH TUTORIALS!!!