1. Restart GMOD.
Fixed? If not:
2. Start your own multiplayer server and join the server you want from there.
Fixed? If not:
3. Close steam and go to your Steam directory ( Not Steamapps. ) and delete "ClientRegistry.blob" and let Steam update. Then do step two and most issues should be fixed.
This is mostly what I would've suggested aswell, as I've learned about these techniques through personal experiences, but the information was taken from another forum. Credits go to Genesis Roleplay (GRP), Niko (A GRP Senior Admin who posted the information), and Lewis (Iminiman, a Senior Admin on GRP, who sent me the information and allowed me to use it).