Hi John,
Congrats to your f-i-l, great choice! As far as gross vehicle weight rating-the 6500# rating seems to be about average with a 150.
The rating has nothing to do with trailer weight other than the weight that is on the hitch must be included in the gross weight of the vehical. If the truck is 5000# and the occupants with gear is 750#, and the stuff in the bed is 500#, this would total 6250#. Now you have appx. 250# left for tongue weight. What I would consider more import is what is called CGVWR-Combined gross vehical weight rating. This is the combo of truck, equipment, and trailer. If it's 12000# and the truck weight is 6250#, the trailer could weigh as much as 5750# and not exceed the rating.
Usually the rating is in just trailer weight and tongue weight in #'s for lighter duty trucks.Hope this helps with any fears of "Low" GVWR. Chuck P.