AA has about 110 daily departures from LGA split between mainline and mostly 2-class RJs, and occupies concourses C and D of the central terminal. The combined US-AA won't be giving up any real estate at LGA, unless Parker has a plan to give away even more LGA slots to Delta or another carrier. Eventually, maybe AA can persuade UA to move to the US gates in Terminal C in a gate swap that would give AA more of concourse C gates. That will probably happen as part of the re-building of the central terminal.
I count 63 US departures from LGA. I doubt there's room for 110 more at the US gates.
At BOS, AA has about 31 mainline departures and operates from several gates in B North. IMO, the merged airline needs to grow in BOS, not shrink.
The merged company will have surplus gates at JFK, DFW, ORD, SFO and MIA, among others, as US will vacate its small footprint in those airports in favor of the AA terminals.
Assuming that the merged airline has to relinquish some DCA slots (a safe assumption, given the government's view that nobody should control more than 50% of DCA, and US is already there), some gates will need to be surrendered to whomever wins the inevitable DCA slot auction - dunno whether giving up some US or AA gates there would make more sense.