  We derive a gravitational block formula for the supersymmetric action for a
general class of supersymmetric AdS solutions, described by GK geometry.
Extremal points of this action describe supersymmetric AdS$_3$ solutions of
type IIB supergravity, sourced by D3-branes, and supersymmetric AdS$_2$
solutions of $D=11$ supergravity, sourced by M2-branes. In both cases, the
branes are also wrapped over a two-dimensional orbifold known as a spindle, or
a two-sphere. We develop various geometric methods for computing the
gravitational block contributions, allowing us to recover previously known
results for various explicit supergravity solutions, and to significantly
generalize these results to other compactifications. For the AdS$_3$ solutions
we give a general proof that our off-shell supersymmetric action agrees with an
appropriate off-shell $c$-function in the dual field theory, establishing a
very general exact result in holography. For the AdS$_2$ solutions our
gravitational block formula allows us to obtain the entropy for supersymmetric,
magnetically charged and accelerating black holes in AdS$_4$.
