Why does GMod immidiately load a background map when it starts up?

I have garry's mod 10 (legit, from steam.) Works perfect. But, since accidental installing the joutomaa hl2 mod in the gmod/gmod folder (a stupid thing to do) every time the main menu loads, the damned thing loads a map, which seems to be the first background in hl2.

I have tried going through the files that were in joutomaa, like the gameinfo.txt, to see if I overwrote the gmod version of the file (a command inside which, on menu load, changes the map to background??) but nothing fixed it.

Which file is the problem, what command line am i looking for and how do I fix it?

I have a slow computer takes about 5 mins to load maps, so this is adding 5 minutes onto the time it takes me to start gmod, which is NOT good.

And please, no nonsense answers like "poop" or "fish", please try answering the question.