I think "fashion" here does not refer to clothing, but to custom. It's saying Gary wasn't actively choosing or controlling his destiny, but events and customs changed and he just happened to get carried along. Perhaps something he was already doing became cool or fashionable and it changed his life for the better, but it wasn't the result of anything he was doing intentionally, just coincidence or luck.

Fashion can have many meanings- not just referring to the style of clothing. Used as a verb, it can mean to put something together. Example- He fashioned the cutting board from oak wood. In this sentence I believe it is used as meaning "plans" or "way of doing something". Gary thinks that maybe he is a master of his destiny, but really the plans around him are just falling into place the way he planned.