Yes, the 747. The Jet has been parked at GTA airports since San Andreas, and Trailer 2 confirms the widebody airliner exists in GTA 6. Game files list a 38,000 kg mass, comfortably the heaviest datamined figure in the confirmed lineup. Rockstar has not said what, if anything, players can do with it beyond admiring it from the tarmac.
You almost certainly will not buy a 747, and Rockstar has not said if the Jet is flyable in GTA 6 or simply airport scenery, a role it has played before. Its Trailer 2 appearance confirms the plane exists, not what players can do with it.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.