Wait, the map went on Garry's Mod on your computer? After you subscribed to it from the TF2 workshop? How does that make any sense? So you're telling me that you can play on this map in Gmod?

I don't know why.

Sou

When you get a source engine game you can mount it to Garry's Mod by clicking on the little controller-icon on the games menu screen. Some games like Team Fortress 2, Half Life 2 and it's episodes and Counter Strike Source will automatically be mounted and ticked because GMod was made for such games.

I think that's why Litronom doesn't bother making GMod ports of his TF2 maps because GMod allows you to play them anyway.