g g has the same index number as the Yellow MissingNo., and shares its sprite too, which also has a high tendency to freeze the game.
g g's name is not recognized through the game properly, causing the user to press the A or B buttons multiple times while using g g unless it is given a nickname. At Level 201, g g will evolve into another glitch Pokémon called ゥ▾ ゥ♂.
This glitch Pokémon article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.