source: http://www.garrysmod.com/terms/
Injecting/Modifying .dlls are against GMOD's TOS, and also against the server's rules.
It doesn't matter what it does, you are more than likely bypassing sv_cheats/sv_allowcslua to do it, which can lead to bigger problems(such as running clientside lua files).
This happened to someone not too long ago, if you want to see how it turned out here's the link: http://forerunnergaming.org/forums/showthread.php?tid=5462