I've done a bit of showing over the years but I've only ever been on the fringe of things, so I can only comment from that point of view.
I've always been realistic about the quality of my various dogs and aware of my fairly inept handling skills

so I've rarely been surprised with my placings, although sometimes a bit disappointed for a few minutes. If my dog and I have had an enjoyable day with friendly people to talk to, lots of nice Cockers to admire, yummy refreshments, maybe a bit of shopping and have come home with a rosette then that's enough for me

I've shown at Open and Championship Shows and on the whole other competitors have been friendly and helpful. But I've heard enough bitchy comments and seen a few arguments, so I know there's more than friendly rivalry out there. The nastiest comment made about me in my hearing was at William's first show. A well known exhibitor, whose grooming table was set up next to mine, stared at William and said very loudly to her friend "Well I don't like THAT". Thoroughly nasty and unnecessary, but I chose to ignore her and just had a quiet chuckle to myself when she had a bad day with her Cockers and Will was placed in both his classes

I think people who show a lot are generally breeders as well, so maybe feel the pressure to do well. There's nothing wrong with rivalry and competitiveness as long as people can lose with good grace as well as enjoying winning. Overall I've seen more good behaviour and met more nice people than nasty, but sometimes the nasty ones are the ones who stick in your head longer.
I also think it varies from breed to breed as well. You mentioned GRs and a friend of mine showed her GR dog for a while but found the GR showing world less friendly than those who show Irish Water Spaniels, which is what she shows now.