I owned the GSA. And presently use the E&S DJR400 (it has an isolator built in, just not sure if it sounds the same as the stand alone unit). I can say that both are very similar as far as the overall sound goes. The iso on the djr400 has a little more gain on it (+12) as opposed to +8% on the GSA, although i should say that the +8 seemed perfectly fine, plenty of gain. One big plus about the GSA is the pointer knobs! i cant stress enough how nice it is to "feel" where the knob is rather than having to look down to know if you've returned it to center. Other than the cost, i dont know why Jerome wouldn't put pointer knobs on his ISO (and mixer!).
If i was in the market for a 19" ISO, i'd pick the GSA over the E&S for that reason alone.