Mind you this is my opinion, but the situation Gabbert is coming into, paired with the situation he was coming from and his draft stock, it seems that Philip Rivers is the best comparison. Hear me out first, naysayers.
In a draft where technically Eli Manning was drafted first overall, but traded in a deal that sent him to the Giants and Rivers to the Chargers, Rivers was not the highest rated quarterback in the draft. Frankly, some of his mechanics as well as his pass-happy offensive system at NC State left many openings for criticism and skepticism. While waiting a couple years for Rivers' time to shine, Drew Brees was becoming an elite quarterback and winning the 2004 Comeback Player of the Year Award. But once he left for New Orleans, the team put Rivers at the helm. So here's how I make the comparison.
While Rivers had questions coming out of college with his side-arm motion, Gabbert has exhibited great mechanics. There's a plus.
At the time, Drew Brees was a starter on the hot-seat and stepped up once Rivers joined the team. Rivers had two years on the bench before he became a starter. When he was drafted he was placed fourth on the depth chart at quarterback. Gabbert will be coming into a situation where, at the least, David Garrard is the incumbent starting quarterback for the Jaguars and that will not change quickly. I see this being a gradual transition, similar to Rivers.
Let's look at the teams. The Chargers had a staunch defense to help Brees/Rivers as well as a stud running back in LaDainian Tomlinson. The Jaguars in comparison certainly have an elite running back in Jones-Drew that can help ease the pressure the Gabbert could face in terms of not having to throw the ball 40 times a game. The defense certainly needs to get better, otherwise the pressure of playing "catch-up" will be too much to put on a new starter.
I would say a few more weapons would be nice for Blaine Gabbert to have, but in the current situation, I'd say Gabbert's situation is comparable to Philip Rivers', especially in the important factor of having an elite running back. This is a hopeful comparison, as Rivers is certainly one of the upper-echelon quarterbacks in the NFL today, so let's hope Gabbert follows the same path.
I will note that is hard to compare any quarterback, as each have their own mechanics and situations that they each come to.