I always believed Roster GSP was a number based on the GSP averages of characters you have played online, prorated so you're not starting at 1 GSP for any new character you pick up. Likely another way to close the skill gap, that way the brand new player just starting with everyone at their starting GSP won't get matched up with someone super experienced and just picking up a new character.
Now considering deranking your characters to 1 GSP is a popular thing to do now on Youtube, and also considering their roster GSP actually goes up when they do this, there may be some other funkiness going on with it raising based on games/hours/whatever played. Or maybe it bases it on the averages of the max GSP that any given character has had at some point, and doesn't take the GSP lowering into consideration. That I'm not sure of.