It looks like Greddy makes two different versions of the TI-C cat back. The regular one tapers to 2.5" where it mates with the downpipe and the Competition version is 3" all the way through. It sounds like you either need a 3" 2-bolt flat exhaust gaket or a 2.5" 2-bolt flat exhaust gasket. If you're not sure what version you have, you might as well pick up one of each size since they're pretty inexpensive. Also, make sure you have good hardware connecting the flanges.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)