Do you have grounds to appeal, no, at least not strong ones. Its the drivers responsibility to be aware of and comply with the contract they have entered into. Unlit signs, didn't know there were restrictions, didn't actually park won't get a PCN cancelled at appeal stage. Claiming forbidding signage therefore no contract won't either but may have some traction in court.
It may say five minutes on the PCN but in reality it will be 5 minutes and some seconds so outside of the consideration period so that won't work. Not being able to see the car in the images isn't unusual at night, the camera picked up the plate going in and out and that's what mattered unless you are disputing being there or your plates were cloned which you are.
You might get it cancelled if you explain you were picking up a gym member who forgot to register you, but it's far from a given.
In reality appeals are unlikely to succeed from what you've given us so far, they will be followed by debt collectors and a letterof claim. There's a good chance it won't proceed to court if it's defended but that means engaging with the legal process later.
Personally I'd be having a long hard conversation with the gym goer, especially if several people's generosity has resulted in PCNs.
