It's not worth looking at FF8 and asking who the tank/healer/dps are in the group simply because of how that game was designed. Squall could easily be a tank, healer, or DPS depending on junctions, typically all three because that game was bonkers in balance.

GNB is a tank because ShB needed a new tank job. GNB could just as easily have been a melee DPS with maiming or striking gear. Squall being Rinoas 'Knight' is the justification we have symbolically for why we have GNB as a tank.