We learn what happened in their fight back when ash and gary were children
Flash back
ash just got done with Gary's father and was shaking in the corner, handcuffs still on.
"NOW. YOU LEAVE MY SON ALONE. OR I'LL KEEP DOING THIS"
ash was silent. the only noise was ash's crying and a few whimpers
ash had told gary to meet him on-top of the pallet town hill to talk, gary arrived next to ash and sat down, they were about 10 at the time
"Gary there's something i need to say"
"no problem ash!"
"we...w-we cant be friends anymore"
"huh?"
"im sorry gary i dont think you understa-"
"no i understand perfectly fine. little ashy-boy over here needs to cut down on his gay."
ash began to cry, he wasnt going to tell anyone what was really going on, never. gary would be the last person he would've told.
"oh so now your gonna cry, your just as pathetic as my father told me you wer-
ash began shaking, turning red like he was going to explode. his hands clamped to his hair and rocking back and forth.
"stupid..." gary spat.
... and that was how gary left him.