I saw the probable answer almost immediately (from 21A + 27A + 55A), but it took a fresh look just before heading to bed last night to see the first GABLE that unlocked the mechanism and prove that my surmise was correct. The title was icing on the cake.
Very clever to create a grid that you could fit them all in symmetrically. (I knew something was up with the unusual grid symmetry, but it's clear in retrospect you needed that layout to fit in the seven GABLES in an aesthetically-pleasing pattern.)