They're swapping gates at LGA, but any mention of DCA gates has been conspicuously absent from the press releases regarding the slot swap. As such, it appears that MWAA, rather than the airlines, assign(s) gates.

DCA's historic terminal building is really cool, particularly that highly dignified lobby. Was that zig-zag corridor in A with the fake window displays (and the AA credit union, etc.) previously gate space?