Well, at least coming from an American background (elementary and junior high school in the USA), GSIS seemed a lot harsher. The teachers there seemed to be more interested in finding ways of criticizing or putting down students than the actual teaching. My home teacher (not sure what it's called anymore, but it's the session you have at the beginning of every day) liked to yell at us, usually when unprovoked. Not sure if it was psychological issues or what, but he would start talking or lecturing, and then work himself up into a literal fit. One time, he very nearly cracked his desk when he put his fist into it. Strangely enough, I saw him at a debate competition about 3-4 months after I had left GSIS. After having spent every morning in his class for a year, he didn't recognize me. He's probably the worst example, and that's not representative of all the teachers. Some were good, and genuinely interested in teaching. But that was one or two out of the whole bunch.
The environment wasn't that great either. Just compared to HKIS, the kids at GSIS seemed to be a lot harsher. Got bullied a lot there, which never happened at HKIS. Could have just been adjustment to a new system, but one year doesn't make all that much difference. Also, just from my perspective, a lot of trust fund kids. Actually, you'll get that at HKIS too, so I don't know if that'll matter to you in the end.
Honestly, after a year there, I knew a little German, a lot about British innovation the past three hundred years, and not a whole lot else. I didn't really learn anything new, and I felt a whole lot more challenged mentally once I made the switch to HKIS.