I have watched the G&L market like a hawk, daily, for at least 15 years. I have been into them, but maybe not as heavily as stalking the market daily, for 30 years. I've bought and sold hundreds.
My observations on the market post-closure are the following:
Most likely, I think we're seeing a lot of uninformed buyers, somewhat new to the brand, who don't really know the company, its historically flat market (especially used), or its manufacturing history (such as the steady cost cutting of the past 25+ years, resulting in its equivalent of the CBS Fender era or the Norlin Gibson era). They're getting the exposure/itch for G&L, and making the same mistakes that many newcomers to the brand have long made: assuming that they're worth more than they are, and overpaying. These shoppers are more willing to move quickly on them, without waiting for price drops or negotiating. They're skewing the market a bit...but not a ton. It's kind of selective what's going up, not all G&Ls across the board. And I don't think it will be permanent.
Average G&Ls from 4-bolt era (6-bolt for basses) have gone up a little, if any. Maybe $0–$200.
More special examples (e.g. flame tops, binding, semi-hollows, etc.) are going for several hundred (up to 500 in some cases) dollars more.
The only things that have gone up significantly are Comanches. Tributes, which should be $400 used guitars at the absolute max, are actually changing hands (not just "asking") at over $700, routinely – sometimes more. I have seen a few fancy U.S. made examples go in the realm of $2,000 used, which is all but unheard of till now. People are enamored with the idea of the split coil pickups (which are basically just G&L Mustang pickups), and freaking out that they'll never be made again – as well as the Comanche having relatively low production numbers compared to the Legacy.
The old stuff (Leo era) has basically remained flat in going prices. The kinds of people "freaking out" about the end of the company seem to largely be people who are into the later model stuff, and don't know or believe that it isn't the best period for G&Ls.
If you're the kind of person who likes to buy and sell instruments, instead of holding on to them for a long time, then I would use this opportunity to sell your hot models (Comanches or examples with rare and deluxe appointments). I would hold on to pre-BBE stuff, BBE stuff up to '97, and average, run-of-the-mill late model instruments. I think this is a bubble, which will eventually deflate. G&Ls have had 45 years to get insanely popular, and it hasn't happened yet. I don't think it ever will – especially when people finally realize that the later model stuff really isn't all it's cracked up to be (and neither are Comanches).
I'm someone who mostly holds instruments once I buy them; I rarely sell any more. So, I will be selling hardly anything during this bubble. Maybe a handful of my ultra clean, somewhat rare later model examples – if people are willing to pay me more than I believe they're worth. For example, I have one of the ultra rare JB MFD models that were released not long before the company died. I saw a set of the pickups alone sell for $700 on Reverb a few weeks ago. (It was former G&L employee Steve Araujo selling them.) So I wouldn't mind cashing in on that nonsense, for something that's basically just a Jazz Bass with hot pickups.