The best way to level up limits (by killing enemies) is in the Mythril Mines or in the Junon forests. Use Matra Magic or Trine to wipe them out - each battle has 5 enemies in them. For the Great Gospel, you don't need chocobos. You can't even breed them in Disc I... do what the others have been saying.

And Great Gospel is SO worth getting. It is SO awesome, even though Aeris isn't.