I know I'm probably late on this, but I was literally having the same problem like a few hours ago. I figured out what was wrong. So I thought I would say what it is for anyone confused as well. Like I'm somewhat new to playing maps on GMod so I didn't understand what it means to mount Half Life 2 Episode 2 in Gmod. I thought it meant to just install the game, but that's not it. In the main menu, in the right bottom corner there is a button that says games and has a controller icon. You click that and a list of games will pop up and you just find Half Life 2 Episode 2 and click that. And that will fix the errors. It did for me.