After 'The Green Sabre' you lose all the territories gained prior that mission. They will no longer be occupied by GSF members, and after CJ's arrival to Los Santos later in the game, you will have to take them over again. There aren't many advantages of taking over the territories prior The Green Sabre other than gaining respect (thus unlocking more gang members available to recruit), money from the asset in the Grove Street, and upgrading your weapon skills.
There are, however, territories which will be owned by GSF even after The Green Sabre, but those territories only exist if you did some sort of territory glitch (it creates new territories in the whole San Andreas). If you are interested, you can check out some topics in the Cheats & Tricks section. I would advise not to exploit this glitch now if you haven't completed the game yet. It's also better to ask whether the particular method for gaining new territories is safe and will not produce more glitches which can seriously damage your game.