For various reasons, I'm the only GM who is consistently active in my DGC's lives, as babysitter when needed and so forth. Sometimes, I admit, it gets to be "too much," as my DD (their mother) turns to me whenever she wants a sitter and some days, I have to say no. However, for the most part, I see it as a blessing cuz of the amount of time I get to enjoy with my grands and the way I'm getting to know them and vice versa. Like PPs, I know I am very lucky.
As far as the opposite issue, Trish, I've actually experienced it the other way, at times, LOL, when the PGPs chose to be more involved. And I would say that's a mixed bag, too. It means there's another set of GPs or another GM to watch the kids sometimes, and that has it's obviously value! Also, IMO, it's wonderful for the kids to get to know and enjoy their other GPs and see that other people actively love them. "You can't have too much love," as the saying goes.
But it can be a problem, too, cuz, due to some things that have been said, I sense they're a bit jealous of DH and me and our relationship with the kids or what they think that relationship is like. And they try to "compete" by getting them the biggest, most expensive gifts and so forth though this may just be the way they are.
It's more of a problem for the parents, however, than for DH or me since the other GPs aren't as reliable (don't always keep the plans) or as respectful of parental wishes as I am. And one of them is prone to trying to instill the kids with their own views without being sure if those are the parents' views or not. Usually, that hasn't been an issue, but once or twice, there was a clash (which may be why they've backed off- far off).