My EASY Sloution
Here's How I did it VERY EASILY
Go to your steamapps folder. (You may have to go into your Steam folder first to find steamapps.) Once your in steamapps, click on your steam username, then click on your garrysmod folder, then once your in there, there will be another garrysmod folder. Click on that one. Once you are in that folder, you must locate the folder titled "screenshots". Click on it and you will see every screenshot image you have taken in Garry's Mod with the "Camera" tool. You may want to save all of these to your pictures, because you will need to delete most if not all of these. If you do not save them to your Pictures, sparingly keep the ones you don't want to get rid of, and delete the rest. Well there you go! Now that you have cleaned up your screenshots folder, you should be ready to go.
(Note: This is NOT a scientific Steam confirmed explanation, and is not guaranteed 100% to fix your problems, but please read below for an explanation of how and why it should work.)
P.S. This worked for me in the same server after I got the error twice before trying this.
EXPLANATION: Ok first, lets look at the situation. We have a problem, and we need to fix it. But to do that we need to know what is wrong. So lets troubleshoot. We have an error. Lets take a look at the error. It says: ERROR Reliable Snapshot Overflow. Ok, Lets take a look at two words here. snaphot, and overflow. We know that these words have something to do with each other. Plus, the fact that these words follow each other, there is nothing in between them. So lets get tune out everything else. "snapshot overflow. thats what we have. It seems to say that we have some kind of snapshot overflow. Oh, look, it "just so happens" that we have snapshots, or screenshots, in this game were talking about, and most of the time were either fooling around with it, or we acidentally take pictures with out little "Camera" tool, so we have a lot of those. A LOT. So many that we had some kind of "overflow" that might cause some kind of "error". Hmm... I know! Remember earlier we were talking about how we had this error for a game with these words? Lets think! If theres an overflow, that means there's too many. So we have to reduce the number. So in this case, there is a "snapshot overflow", so the only way to fix this is to reduce the number of snapshots. The only way to reduce the number of something on a computer is to delete something. Sooo... wait... did I just think? No it isn't possible.. did I? ...wait..yes...yes...I've got it! Reduce the number of snapshots by deleting them! So we have to delete some of the snapshots!
. Get rid of some of them that you don't need.
