List of version exclusives in Firered and Leafgreen

Wait

FRLG goes to Johto?

If I manage to get the money I'm doing both

If not then Fire Red

no but they go to the sevii islands

there is a plot, i believe they have their own gym leaders

That would be cool, but you might be thinking of the Orange Islands from the anime. (Which may in turn have been a partial inspiration for the Sevii Islands.)

The Sevii Islands are the location of a few side quests, primarily focused on Team Rocket and connecting to RSE once you have the national dex (redundant for these releases unless they port them over too). They also moved Moltres to the first island. The events for Ho-oh, Lugia and Deoxys took place on their own islands.

And yes, the Johto Pokémon previously rendered inaccessible due to incompatibility with gen 2 are catchable from Four Island onwards.

No gym leaders, though.

The Sevii Islands represent a lot of missed potential. They exist to plug gaps in missing content and bring in new gen 3 features without changing base Kanto. More obligation than thrilling content.