Floyd Mayweather -- My GF's Next Gift ... Is a PRIVATE JET!

How do you follow up when you've just bought your GF a $300k Rolls-Royce? According to Floyd Mayweather you get her something much more fly ... as in something that actually flies.

Money and his chick "Bad" Medina were out in L.A. on Thursday when Floyd -- the top money earner in all of sports -- told us what Medina can expect the next time he's in a giving mood.

For the record, a Gulfstream G650 (aka a G6) ... reportedly retails for about $56 MILLION.

In other words, that's half-a-year's pay for Mayweather.