One of my favourite places to give blowjobs is the gym, preferably after a sweaty workout.
Remember, in the UK this is considered a public place from the perspective of the law so this has the potential to go bad, at least to the extent of having to find a new gym. Here's how to minimize that risk:
1) Pick a good location. If the changing rooms have a secluded corner that can work and especially if you enjoy the extra risk.
But for me a toilet cubicle or changing stall works best. Shower cubicles are often a bit too open to give adequate concealment. Being petite I can make a toilet stall work but you may need a proper changing cubicle or a disabled toilet. Sometimes the supply cupboard will be attached to the changing rooms and that can work well if it's not locked.
It perhaps goes without saying but *always* use the male changing rooms. It's far easier for me to get out of male changing than it will ever be for a guy caught in female changing. That's 100% going to cause a ruckus and really the only way this goes *really* bad.
2) Know your times. Both for when the changing rooms are going to empty and for when staff clean. Nothing spoils the mood more than a dude whistling and waiting to clean the stall you are both secreted in.
These days it's likely that your gym has an app that shows how busy it is which makes it really easy to find good times in the off-peak when they really aren't busy. Of course people are still going to be using it so you always have a good chance of being interrupted.
3) Relax. When doing something illicit like this it's normal to feel like everyone guesses what's going on. Most of the time you're going to get interrupted. Someone needs a pee, people changing, even at the off-peak. It can be unnerving, but, unless you're making a scene, nobody guesses.
Keep him in your mouth but slow right down and maintain eye-contact to steady him. Smile. Be cool and enjoy what you are both doing.