And maybe Hermann Weyl. It's as usual: His math is brillant, also from a didactic point of view when referring to his very famous textbook "Raum, Zeit, Materie" ("Space, Time, Matter"). The mathematicians of his time, however seem to have thought not so positively about this book, because in Heisenberg's book "Der Teil und das Ganze" you can read about his experience with the famous mathematician Ferdinand Lindemann, whom he consulted concerning the choice of his subject of study at Munich university. When he told Lindemann that he has already read Weyl's book, Lindemann told him that he is already spoiled for a serious study of mathematics ;-)).You might want to reconsider that since very few prominent specialists in GR have been professional mathematicians. The only one I can think of off the top of my head is Roger Penrose.
Weyl's physics is not that brilliant, because the idea to gauge scale invariance of the matter-free gravitational field and taking the corresponding gauge field as the electromagnetic field was immediately considered wrong by Einstein and also Pauli, because indeed the measures of rods doesn't depend on their "electromagnetic history". In any case this idea of "gauging of symmetries" was ingenious in its own write. Weyl simply gauged the wrong symmetry in this case, and the entire thing gave the name associated with this idea till today: "gauge theory", "gauging a symmetry", etc.