I believe in some communities or other countries it's common to leave a dog tied outside an establishment but in my area you would never see that. I would never do it. If I was a customer at the gym I would be pretty peeved to have to pass a lunging dog with a absent owner. You have to know you are setting yourself up for a potential negligence lawsuit.
You are not there to immediately correct this behavior - so the dog continues to do it and it's spilled over to lunging when you are present. He probably finds the action rewarding - he's his own boss.
I would get back with the trainer but would not recommend sending him away to the trainers - you need to be present for all of it. I'm not sure how the dog views you but the bond needs to be fine tuned.
I'm sure others will chime in. Good luck.