It takes a bit of work, but it's possible to see Gilliam as the first of the postmoderns rather than the last of the moderns.
His installations, which sometimes mix draped canvases with raw construction lumber or handcrafted, hand-distressed sawhorses, can feel strangely theatrical. They feel like what a stage designer would come up with for a play set in a heroic modern artist's studio, rather than like the real works of art that would come out of such a studio. That is, they feel like references to modern art, rather than abstract work meant to speak on its own terms, in isolation from the world.