Ok so I look at other gym moms similarly to how I've looked at co-workers. They're a huge part of your life while you're at the certain company/gym. Some may even become true long-term friends over time. But the majority will fade away as soon as you leave.
I also tend to treat them similarly. Like I don't tell a coworker everything, I don't tell other gym moms everything. I wouldn't vent about a boss to most coworkers, like I wouldn't vent about a coach to most other gym moms... Like others have suggested, I come home and complain to DH.
Though on rare occasions I've checked if they're experience is similar, etc, or used ones I trust best as a genuine sounding board.
One family I had sworn we were good friends with, and our friendship had grown over a number of years... Well, I've spoken to the mom once since her DD quit four months ago, and texted a couple of times. All efforts were initiated by me.
That was hard, but a reminder, I guess.
And I do follow a few other kids from other gyms that DD has trained with, but it's more genuine curiosity. I'm also friends with their parents on Facebook, etc, so it's less creepy maybe? One just fought her way through a big injury to recently finally compete again - I had genuine tears in my eyes when her mom posted pics. So I think sometimes it is normal and ok, but know it can also get obsessive. ( and I am also Facebook friends with a few whose kids have moved onto other sports, and it's fun to see their progress... And it's informative, should my DD move on!)