Well, the best measure of what a gun WILL sell for, is what that gun IS selling for. I looked over on auctionarms.com, and gunbroker.com. A GP 100 seems to go for between $350-$400. Now, that is retail- on trade in expect that to be cut by 25-50% (dealer IS looking to make a profit)
You may check around, and see what you are offered AS A TRADE-IN- if too low, sell it, and buy what you want.
For a fun 22 plinker, you might look at the Ruger 22 autos.