Prenyltransferase specific for geranyl diphosphate as prenyl donor and coumarin as prenyl acceptor. Can use umbelliferone and esculetin as substrates, and with a lower activity, 5,7-dihydroxy-coumarin and 5-methoxy-7-hydroxycoumarin. No activity with 5-hydroxy-7-methoxycoumarin, bergaptol, xanthotoxol, p-coumaric acid, caffeic acid, 2,4-dihydroxycinnamic acid, kaempferol, genistein or homogentisate. No activity with dimethylallyl diphosphate, farnesyl diphosphate or geranylgeranyl diphosphate as prenyl donors. (2E)-geranyl diphosphate + umbelliferone = 8-geranylumbelliferone + diphosphate (2E)-geranyl diphosphate + esculetin = 8-geranylesculetin + diphosphate Belongs to the UbiA prenyltransferase family.