http://www.pcgamer.com/gta-5-mod-adds-the-doomsday-heists-jetpack-to-single-player/

Last month, GTA Online introduced its three-part Doomsday Heist sequence with new objectives, rewards and toys to muck around with. Sam and Tom have since discussed the merits of flying cars, while the update also brought with it the Mammoth Thruster jetpack. 

The latter is now available in Grand Theft Auto 5's single player by way of jedijosh920's Jetpack SP mod.