I think that, in theory, the fear is that the gelatin would pull the suspended yeast out of suspension, thus giving you flat beer. But I don't think that gelatin pulls out 100% of yeast - and it is only really effective at cold temperatures anyway. I know British brewers add priming sugar and isinglass at the same time to carbonate and clear their kegs - I suspect the same would work fine with gelatin. That said, I have never tried it. I would sooner use gelatin in the fermenter (along with a cold crash) and then bottle.