sonyeoshin is correct; there are no bus gates at IAH. Your flight will arrive at D or E because that's where all non-pre-cleared international arrivals go. Most, but not all, international UA departures will be from E; a few are from C (and you'll, rarely, see some from D). The Minimum Connection Time for UA-UA International to International at IAH is 1h10m, so this is a very aggressive plan. The fact that you don't have to wait for your bags is a plus, but you will have to go through immigration, customs, and security. (Even though you don't have to wait for your bag, you're still going through customs, and you still need to follow all US entry regulations).