So I just tried a couple shots with BEEMAN perfect rounds (not GAMO, sorry) in my Walther PPK.
Right away, before shooting, I tried to roll one down the barrel. No go. The round ball is too big (and the PPK barrel is LOOSE, too).
I put two rounds in the magazine and fired into the snow outside my back door. It worked fine (I kinda thought I was going to get a PFFFT-hiss out of it and a jammed barrel, but it worked okay). I did notice that it worked the magazine apart a little at the top, but it was pretty flimsy to begin with.
I think it actually shot HARDER, which isn't hard to imagine, considering the PPK only gets up to about 270fps to begin with.
I guess you can try it out, just know that you'll be putting unusual stresses on the barrel and it's anchor. But worst case, I think you'll just jam it, and that's fixable. I'd check every couple rounds to see if you notice any damage to the magazine, or if the barrel is being beat up by the lead ball slamming into it's rear end :)
If you do play around with it, post some results! -JC