What is a 'gong fu brother'?
Well, considering that, at least in the past, the children that were deposited at the Shaolin temple became "family-less" rather on a sudden basis, as they were dumped into one of the pavilions inside the temple walls, to be, at least for the next five or ten years, under the tutelage of a monk master. Any semblance of familial brothers or sisters was gone, as well as the previously well known "mommy" and "daddy". These kids were on their own, separated from the rest of the world, and isolated from it, by the walls of the Shaolin temple. They were well taken care of, though the discipline was rather harsh, if it was necessary. The children were grouped together with other children of similar ages, and together, they grew up, and learned the skills necessary to become Shaolin monks in the future. Because of the lack of real family, the other children in their groups became their "gong fu brothers". One family unit became replaced with another.
What I've found interesting, is that the strength of the bond between the gong fu brothers (and sisters) seems to be tighter and stronger, in some cases, than actual family ties. The meaning herein, is obvious: either the original family ties were weak to start with, or, some major family bonding was absent during the very imprintable years of six to ten onwards.
The term gong fu brother is now used outside of the temple walls. You can find it used rather freely in most of the schools in the village. It basically refers to a "classmate", though, because of the rather harsh conditions the "classmates" must all endure, the bonds between the gong fu brothers in a school are rather strong.
Which leads me to a few words of advice. If you start a fight in Shaolin with one guy, you've basically started a fight with his entire school. And if the kid goes to the Tagou school, which has over six thousand "gong fu brothers", it's going to be a very, very long day.