Presently it appears that there is a qualitative difference between gas gas giant planets and brown dwarfs: brown dwarfs form by gas accretion in cloud condensation cores, gas giants however are seeded by dust-to-rock coalescing in discs (the same process forming rocky planets like ours and icy ones like Pluto or Halley's Comet). An evidence is that very few brown dwarfs orbit stars. I don't know if there is a hypothesis explaining the upper mass cutoff for planets; on the other hand, cloud condensation cores under 13 Jupiter masses would be possible theoretically, so there is a terminology gap.

_{*Lunatic*, n.
One whose delusions are out of fashion.}