it's sad as a huge Portal fan, but at this point Valve's been a platform holder longer than they have been a "console games" studio. The only stipulation is that they are free enough with their work that you have a 1% chance of turning your own Portal/half-Life Mod into a sanctioned release if you impress them enough.
I'd say "be the change you want to be", but I think the worst use of your talent is investing in an IP you do not own/have a license for. Every Garry's Mod has hundreds of broken dreams underneath. And I especially hate this line, but: If you're gonna mod you need to do it as a hobby. Because monetizing it is the easiest way to get on 99% of companies' radar.
Then again, I could have just said offline games, and maybe I was overthinking it.