I've been both a 'non-paid work' school gate mum, and a professional working 'drop and run' school gate mum. It's quite ridiculous to turn your issue into some kind of tribal warfare.
When my kids started school I already had friends who I'd known since toddler group. So of course we'd chat together at the end of the day while waiting for the kids. It's perfectly normal behaviour, surely? We didn't gossip or judge anyone, we just chatted for ten minutes. We weren't a 'clique' we were just people who knew each other and had to stand around in the same place for a while every day.
When I went back to work and had to drop and run, and pick up from a child minder rather than from school, I missed the chance to catch up with people, and as the kids grew older of course we all saw a lot less of each other. I couldn't have cared less about what the later bunch of less rushed school gate mums did. Why would I?
It's just a phase of parenting. If you stay home with your kids for any period of time, you have the ante natal bunch of people, the toddler group friends, the nursery/playgroup people. They're just people you're thrown together with, just as you are with work colleagues. If you choose to, or have to go back to work when your kids are small, then these people will not be your tribe. That's neither their fault nor yours.
If an individual is rude to you or ignores your approach to them, then they're a rude person. They don't represent every non-working school gate mum any more than you represent every working one.