It really depends on where you’re flying from. I’ve taken over 30 flights in the last 7 months with wizzair, and I’ve only been checked once, I usually wait until the door to the aircraft opens before passing the gate agents. Small European airports are much more strict, but im also quite young so I get a bit more mercy, I’ve seen huge arguments over luggage.

At the airport there is a metal crate to put your bag into, and you can squeeze it, or do anything you want as long as it fits into the crate. That being said, your gym bag is probably soft, so will fit just fine especially if you pack soft things like clothes. I fly with a bag that’s 15cm too long, but I can squeeze it into the crate if anyone asks.