OBSOLETE. Catalysis of the reaction: GTP + H2O = GDP + phosphate. GTP-hydrolyzing enzymes, where GTP and GDP alternate in binding. Includes stimulatory and inhibitory G-proteins such as Gs, Gi, Go and Golf, targeting adenylate cyclase and/or potassium ion and calcium ion channels; Gq stimulating phospholipase C; transducin activating cGMP phosphodiesterase; gustducin activating cAMP phosphodiesterase.