yes you can go back to gingerbread if you dont like ics you just have to download the stock tar file from xda or sammobile and flash it to your phone via odin.
it said the same thing with mine when it was on 2.3.6 gingerbread i flashed 2.3.5 back on the phone and it let me update. you can try to uninstall and reinstall kies first see if that works.
i had to flash 2.3.5 gingerbread back on mine before kies would work mine was saying the same thing when i was on 2.3.6. you can try and unistall and reinstall kies first.
seems like a very stable build love the battety life way better than the last leak. I dont use gps so not sure bout mine. So happy i got ics on my skyrocket got tired of waiting on att and samsung.