Go to the summit of Mt Moon and throw away a Moonstone. A Meowth Balloon will come down and a man in a black hood will get out. He will battle you and will have six Mewthree. After you beat him he will reveal that he is actually an alien and is the reason that Clefairy came to Earth. He will battle you again with the same team but halfway through the volcano turns active and explodes, raining magma everywhere. The man in the hood will try to escape in the balloon but it will be caught in the magma and die. At this moment you have to go into your menu and have a Pokémon use a PP Max Protect, and it will defend you from the magma. Once that's done Gary's Dad will come out from the volcano and congratulate you on defeating his son, but you won't know what he means. He'll tell you to return to Professor Oak. (From now on he'll be the Champion) You go to Oak and he'll be in grief that Gary has died, and you'll discover the man in the hood was Gary all along. He will let you accept the last Pokémon he was going to give away; another of the starter you chose.
If you want a Bulbasaur you'll have to gen one or trade with someone else.
