For a free mod that has been going for 10 years now and has been largely praised by the modding community, I personally think it would be in poor taste to do something like that. The backlash at this new paid system Steam has implemented has been pretty strong. Whether you agree with this new system of theirs or not, there's no denying people are largely upset.
I personally think the implementation of it was poorly handled. Under the guise of "helping the modding community", the modders only get a 25% share of the profits, the rest going to Steam and (in this case) Bethesda. They say they will implement a "donate what you want" method also, but time will tell what happens. But I'm going off track here.
People would pay for assets in Garry's mod, to discover that those same assets have been offered for free as part of GE:S. I don't think it would do anything positive for the mod. Not to mention any legal issues there might be to dance around, which have been cautiously avoided for so long. But I'm no expert on the legalities of these things, so someone else could probably answer that part better than myself.
There's been tons of time and great work put into GE:S, and yeah it would be great to see people compensated in some way for that. But this new Steam system is a bad idea, and to risk being labelled as sellouts just to make a few quick bucks on Steam is the wrong way to go.
Anyways, that's my 2 cents on it.