I'm gonna take a guess and say this has to do with the region you are in and GMod not searching for servers outside of your region. Could be wrong. You can always just use Gametracker to browse Garry's Mod servers as an alternative.
You can try to type sv_region "255" into console and refresh the server list. That's all I can think of sadly. There is always the factoryreset.bat in the garrysmod root directory that can reset your game and such that fixes a lot of issues.