All you have to do is look at the truth table to see what conditions set GS low and EO low. You can also see the effect of the EI^ input (hint - when it is high, what happens to the outputs, including GS^ and EO^?).
GS is "Group Output" - goes low on priority detect, with priority address in A0-3, also active low, combined with GS to flag which IC when cascading two.