A script is creating errors - there's usually a few of reason for this:
It's not your fault. There's nothing you can do. If you're feeling kind you can tell the server owner that their server is erroring out. If not - just leave the server and never join again.
If you're not joining a server - and the errors are happening on your local machine - the best way to fix this is to clean your GMod installation. This will delete everything and restore your GMod to how it would be if you did a fresh install. Your workshop addons will all automatically be downloaded.
If you don't want to clean install - you need to look in the console to see which scripts are creating errors, then go into your "garrysmod" folder - and try to find the script that's breaking. You'll want to look in "addons", "gamemodes", and "lua" folders for the errors.
Do what it says on the page. Get rid of the scripts. I know it's annoying - an dI could easily hide these errors so you'd never know they were happening. But they'd still be happening under the hood - and then you'd be annoyed by that - instead of this informative message.