In theory it should work, please

1. Check if GBGMC is active in "Modding" tab
2. Close game, right click Properties in Steam, see if GBGMC shows up in Workshop tab.
3. Run "Verify integrity of game files"
4. Start the game again

I will try that but it showed up before in the mutilators but didn't show up when trying to start a game. Would using this mod with yours cause an issue? https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3266106780 ?
I tried unchecking this mod and your mod still didn't show up. Thanks for your mod and your help. It adds so much to GB.

1. Check if GBGMC is active in "Modding" tab YES
2. Close game, right click Properties in Steam, see if GBGMC shows up in Workshop tab. YES
3. Run "Verify integrity of game files" Done, no errors.
4. Start the game again. I am thinking these mods are overlapping and I should use one or the other. Am I right? Not clear which game type is yours or the other one.

In game/properties/Workshop, both of these mods show up but the other mod (first listed below) shows 21.21 MB installed. Your mod (second listed below) shows 0 MB installed. This must be the problem?? For some reason it's not installing from Workshop even though I subscribed and it shows up in-game.

Mod
GB Checkpoint, Outpost and PVE modes
CHECKPOINT style game mode. This shows up.
OUTPOST [First pass for compound map] This shows up.
ADVANCED TERRORIST HUNT This shows up.
INTEL RETRIEVAL TJL Mod This shows up.

Mod
GBGMC Game Mode Collection
Asset Extraction This does not show up.
Break Out This does not show up.
Break Through This does not show up.
Kill Confirmed This does not show up.
Security Detail This does not show up.

OK, I got it work. I unchecked your mod in properties/workshop. Then checked it again. It said it downloaded a file. Still shows 0 MB in properties/workshop. I restarted Steam. Still shows 0 MB in properties/workshop and still not showing up in the game. At the bottom of the properties/workshop screen, it said 1 file downloaded. I clicked on it and it took me to a screen I haven't seen before with a green button "play" Ground Branch button. The game launched and your game types showed up in the game. I backed out and checked again in properties/workshop and it now shows 7.63 MB installed. For some reason your mod is downloading but not installing unless you follow these steps. Hope this helps.