OBSOLETE. Catalysis of the reaction: GTP + H2O = GDP + phosphate. Small monomeric enzymes with a molecular mass of 21 kDa that are distantly related to the alpha-subunit of heterotrimeric G-protein GTPase. They are involved in cell-growth regulation (RAS subfamily), membrane vesicle traffic and uncoating (RAB and ARF subfamilies), nuclear protein import (RAN subfamily) and organization of the cytoskeleton (Rho and Rac subfamilies).