Wikipedia post (thank you wikipedia) with this diagram about G protein, G-protein coupled receptor GDP and GTP I thought was pretty good. “Activation cycle of G-proteins (purple) by a G-protein-coupled receptor (GPCR, light blue) receiving a ligand (red). Ligand binding to GPCRs induces a conformation change that facilitates the exchange of GDP for GTP on the a subunit of the heterotrimeric complex. Both GTP-bound Ga in the active form and the released Gßgamma dimer can then go on to stimulate a number of downstream effectors. When the GTP on Ga is hydrolyzed to GDP the original receptor is restored.”