I got "Invalid Steam Key" or something similar!

Well, it's not your Garry's Mod, Steam, PC, etc.
You'll have to blame Garry for the update.
If i'm not wrong, the admins are fixing it. (Right Magik?)
So what you should do now is just wait.
DO NOT WHINE OR TELL US TO HURRY UP!

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if you cannot find a certain rnd server and you have at least one rnd server in ur favourites, join that one you have. then type !servers and join manually. another way you can go on ur desired server is to get the ip of one you want to go on and type in console:   

connect SERVER IP HERE