Steam overlay broken only in gmod?

Well the topic sums it all up, it works perfectly fine in Team Fortress 2 and in the games I added via Add a Non-Steam game, however when I press Shift + Tab in Garrys Mod, I dont get anything at all. I am running Malwarebytes Anti Malware and Avast Antivirus Free Edition in the background, if this might somehow affect it, though I doubt it seeing as how the other games work 100% fine. Do I need to flip some kind of in-game setting in Garrys Mod for this to work? My steam friends are saying it works fine for them and Im gettin real sick of alt tabbing to reply to them.

I am not sure what could cause this, but a good workaround would be to open Steam, go to the game list, right click on Garry’s Mod, go to Properties and click Set Launch Options. Paste this in: -windowed -noborder and you’ll be able to alt+tab out of Garry’s Mod without it getting rid of the window, causing lag, or crashing your game out. I use my in-game browser but most of the time, the in-game browser just doesn’t do what I want it to do, so if anything you’ll still be using your actual browser, and this comes in very handy! Makes switching windows a breeze.

Off-topic, but… THANK YOU SO DAMN MUCH I THOUGH ONLY L4D AND A FEW OTHER GAMES HAD THIS I LOVE YOU.

As for the original topic, I have no idea, sorry.

Go to the games properties and make sure that the "Enable Steam Community In-Game is checked. Hoped this helped, it worked for me.