If we are just using negation, conjunction and disjunction, then a positive occurrence of a subformula is one that occurs behind an even number of negations while a negative occurrence is one that occurs behind an odd number of negations.

Ga on slide 84 has a positive occurrence, and as slide 82 explained, we can neglect fairness constraints for [phi WU psi] (means for all weak temporal operators since all of them can be reduced to WU, and since we have G(a) = [a WU false], it also applies to G).