GTA's dirt bike of record, the Maibatsu Sanchez, is confirmed for GTA 6 through Trailer 1 and screenshots. It borrows from Japanese 450cc motocross machines like the Yamaha YZ450F and has been in the series since Vice City. Files list 220 kg and four gears, both datamined. Expect the usual Sanchez deal: quick over dirt, jumpy, and easy to replace when things go wrong.
No confirmed method, but the Sanchez has never been hard to get in any GTA game: dirt trails, backyards and rural spawns have always supplied them freely. Expect the same, with the caveat that nothing is officially announced.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.