I've never been on these servers before, and my spray is automatically overridden when used. Why might this be?
press f1 and go under spray and put it there, we have a different spray system than the default gmod one so thats why
SGM has it's own way of dealing with sprays entirely separate from the Garry's Mod way. Next time you are in game, press F1, and go to the "Spray" tab. That will give you directions on how to use your spray on the servers. Although, from the looks of your profile pic, I don't think I want to see your spray.
Since no one answerd your question, Default Source sprays used to have a potential to be exploited (running server commands on spray), that's why it was removed in the first place. (It might be patched now though) SGM sticked with SprayURL due to them having the ability to be changed client side (turned on or off, animated sprays or not), administrative control (showing links, removal, animated or not) and it doesn't have any exploit risks.