Yes, by the standards of polling, Gallup is one of the most reliable, although Rasmussen, Zogby, Marist College, Ipsos, and Pew are reliable too. It's basically impossible to claim one organization as more reliable than the others, because they consistently change their polling methods. For example, Rasmussen, in the 2004 cycle, did 3 day rolling averages, a new approach which turned out to be very accurate.
It'd be good to remember that in the 2004 election polling, of the 14 major polls, Gallup was the 4th worst in accuracy of results. Pew, TIPP, and CBS topped the list.