Have you considered cross-grading them by academic subject? Mainly, you would need to have an appropriate section of each subject available at the same time as the regular level of the subject. E.g., if you have a fifth-grader who can do sixth-grade math, then during their team's math block, cross-grade them to a section of math when any sixth-grade team is having math (it doesn't have to be their home team).
You have only two grade placements in the building, so you could also just place everyone by instructional need (up & down) (perhaps use your end-of-year progress monitoring data from last year), which will likely create teams that are mostly-fifth, mostly-sixth, and a couple of mixed 5/6 teams. (The majority of kids will still sort into their nominal grade.)
The pull-out GT teacher would then take on more of a consulting/inclusion role, rather than a subsep enrichment role.
