My two BG do similar stuff to one another, often flaring their gills, all their fins standing out full, and they race towards each other like they're playing chicken. It does sound like a territorial or dominance display, but I'm an absolute amateur - actually I'm even lower than that - when it comes to fish behavioral issues! It's cool to watch, though, and try to figure out what's going on.
I really believe, and would like someone more knowledgeable to weigh in, but I really get the impression they're almost playing, like they're happy after they've eaten and are full and more social. That's what it looks like to my very untrained eye.
