How Do I Set My Invisibility in GMOD Server WIth a Mod

I solved the problem. And the culprit was * drumrolls * ...

corrupted mod files

I downloaded it again and now it works. Thanks anyway folks!

Another tip for the future for vanilla players who still use F3:

When you die, do not respawn. Instead quit to the main menu. Now open up a different world and hit F3. Now quit that different world, and go back to your main world. You'll still be dead, but now you'll be able to see your coordinates. Hit F2 to take a screenshot and you can then respawn and run to the coordinates that you've screenshotted.

Note that the items you've dropped will not despawn until they're in a loaded chunk for five minutes (cumulatively). So, if you're a LONG way away, take some time to gear up properly; you don't need to hurry until you're close enough that those chunks are loaded again.