"Oh. Yeah. That's it. I'll call in the troops, you wonder where Perry is."
The Gang is a group formed by Phineas, Ferb and their friends. The group is not consolidated until the second season and the main members of the gang are Phineas, Ferb, Isabella, Buford and Baljeet, with occasional participation of The Fireside Girls and Irving, but very rarely with other characters (ex.: Jeremy and Django).
In the first season, it was weird see the full gang complete; generally, Phineas and Ferb act alone, sometimes with the help of Isabella and the Fireside girls. They also tend to do stuff with Baljeet in a few episodes ("Unfair Science Fair"), and the same with Buford ("Raging Bully").
In the second season, The Gang, is seen as a more consolidated group of friends; in this season, Phineas and Ferb start going less with their family and going more with their group of friends. The incorporation of Irving ("Hide and Seek") to the group marks a change in the group's dynamic, besides not going with them as much as the others and not been considered a "friend" by Phineas until "The Doof Side of the Moon".
Seasons 3 and 4
The gang is completely consolidated, and it is weird to see an episode in which Phineas and Ferb aren't with their group of friends; generally, they appear in mostly every single episode in those seasons and they work together, also, Irving appears more in this season.