Importing models and/or textures from other source games is illegal, right? Thats why I need some help with it.

You know Garry's Mod, if there is a map where you have missing textures/models, you can't see them unless you buy the game which includes them. <-- Question is there.

How I can make same thing? In map having textures/models from Portal 1. That means you need Portal 1 game to see all textures/models in Portal 2. Or is there any other ways to do it not illegal?

Thanks.

Make a modified version of it.
Or ask the model author for permission if you can use it.

At the very least, I guess you could say when you release it that you need Portal 1 to play it. It wont stop anyone from using it, but I suppose it could giuve you some sort of leal grounding.

You should have got Portal when it was free that one day...

Buy Portal?