So, every time I try to start a singleplayer game I get this error.

Some kind of clarification would be great.

i would check for updates on gmod… otherwise i think someone may have shot the server with a 12 gauge =P

It’s fully updated and I have yet to find any shotgun pellets around my router.

Put a metal box around your router, that will prevent any kinds of shots.

and I’m still getting the error in singleplayer, multiplayer works fine.

Buy a kevlar vest and helmet for your server.