Garry's Mod Error: My last hope...

I am getting this error in GMod: "Error loading gamemode: !IsValidGamemode".

I'm not sure if you guys are experienced with Gmod, or Lua, but I'm trying to make a gamemode, and I can't test anything because the engine crashes when I try to run my gamemode with a map.

You guys are my last resort. Google doesn't help, and the forums didn't even ANSWER my post, nor do I expect them to answer back kindly.

The error clearly implies you made a mistake creating your gamemode. You will have to show your work before anyone can help. The error on its own isn't enough.

That said, I have no experience creating gamemodes for Gmod, although Lua is easy enough.

Where can I post it? I don't really want anyone to see it online besides you guys. Do I simply use an HTML tag and paste my code?

Yeah, you could do that, but then the forums are easily browsable by anyone. A tool such as pastebin may give you a bit more of control, but I don't know if it goes as far as hiding your code behind a password -which you should share out in the open anyway, if you want everybody to help you out.