All three of the Kanto starters' final formes has a G-Max form and a Mega-Evolution form, however, Charizard has two Mega-Evolution forms, Mega X and Mega Y, respectively.
Gengar can Mega Evolve and has a G-Max Form
Pikachu and Eevee both has a G-Max Form and a Z-Move. Pikachu has 2 Exclusive Z-Moves, Catastropika and 10,000,000 Volt Thunderbolt, depending on its' forms, respectively. Eevee's Z-Move is Extreme Evoboost.
Snorlax both has a G-Max form and an exclusive Z-Move, Pulverizing Pancake
Hope I helped!