How to properly ground a subpanel in detached building?
Pull 4 conductors (2 ungrounded (hot), 1 grounded (neutral), 1 grounding) (250.32(B)(1)).
Grounded (neutral) and grounding bus must be separate at sub-panel (250.32(B)(1)).
No need for a GFCI breaker in the main panel, unless your local code requires it.
A grounding electrode system is required at the second structure (250.32(A)).