2016+ Tacoma mounts can be made to work, both yakima and thule (shown), but need slight modification.So I finally got to drive a Gladiator today and got to look at the bed and try and answer this nagging question.
In regards to the back rack, the aft most position on the trail rail is 4’6” from the inside edge of the front of the bed. The last 6” is used by the framing for the tailgate, etc. So in theory, with the Mopar cross bars (if/when they are available), would limit to <9’ kayak.
The other interesting thing is with the trail rail installed, Yakima style racks can’t mount to the bed since they clamp around the bed frame (unless the trail rails are removed).