http://info.topasweb.com/BBS/BBSView...String=&page=1
According to this link, they will open the counter up to 40 minutes prior to the flight at "H" island. Do you have any baggage to check in? Regardless, you won't be allowed to check in at Seoul station for CI, so don't expect anything there and rush to the train. Being a Gold member doesn't change anything in terms of cut-off time.
Seoul station is huge with complicated layout. ICN airport is also very far from the train station (in my opinion). So get to know which way to go beforehand. And get CI office phone number so that you can call in emergency.