How to create maps for gmod?

When i open up Source SDK, the only game choice i have, is Portal. Why is that?

Also i have heard that to create proper gmod levels, i would have to make a confic file, that includes all the games gmod is based on. How do i do that?

Refresh game content to get other games.
And garrysmod doesn’t have a set game config. You can use HL2EP2.

Check this page from the Valve Developer’s Wiki:

You could probably use Portal to create maps for gmod.

I guess you cant spawn any hl2 weps then.