Hear me out here.
Ash is Red's son, Gary is Blues son & they are both really bad at naming children.
Red = The color for fire.
Red names his son Ash.
Blue = the color for water
Blue names his son Gary as in gyarados.
But wait, theirs more.
Gyarados is a water type pokemon.
Gary Dos, as in Blue the second.
Blue just wanted to try and humiliate his rival one last time by coming up with a name more clever then Red did for his son.