To get achievement, we need to create script file, if you don't want to tap Q key alot of times. Find your "steam/steamapps/<Your login>/garrysmod/garrysmod/cfg" folder, then create text file. Name it like "menu_user.cfg", open them with notepad and type this:

alias "Qmenu" "+menu;QmenuOff"
alias "QmenuOff" "-menu;Qmenu"
bind "Q" "bind q +menu;Qmenu"

Then save file.

So, we created script file (don't worry about re-binding Q key, script must return it back), then we need to launch it!
Start your Garry's Mod, open any map(but sandbox gamemode) and type in console command "exec menu_user.cfg"(Without quotes), then close console and press Q, and your Gmod will get stuck. Give it a minute, or 10 secs probably will be enough too.
DANGER!: when you will press Q key, your Gmod will stuck, shift+tab or alt+tab keys will not work, the only way to disable Gmod is Ctrl+Alt+Delete! So, when you will close your Gmod, you WILL get achievement. If not, do the same thing, launch Gmod, script and press Q key and wait some time. Also, if script will get rekt Q key, you should type "bind q +menu" in console.