Hello, and welcome!
I'm so sorry to hear that Gucci is not acting himself.
From your description, this sounds like a condition called feline hyperesthesia, which is not uncommon. It's neurological in nature and likened to seizures. The cat's back/tail area become super-sensitive and they try to bite or lick furiously at it. The skin on the back may 'ripple' also, with this condition.
Here is more information:
The panting is of concern, so try to calm him down and stop him from running so much, but don't touch his back/tail area.
He will need to be seen by a vet, to make a definite diagnosis and prescribe treatment. He also may be referred for a neurological work-up.
Unfortunately, there's not much you can do for this condition at home, so I hope you can get him to the vet, as soon as possible to be evaluated.
I hope all will be well with Gucci and please let me know how he's doing. Thanks!
I hope you found this answer helpful and will please take a moment to select a positive rating from the stars at the top of your page, then click 'submit;' this is the only way I will receive credit for assisting you. Thank you very much!
Even after rating, I will always be available if you need additional information; just click 'reply' and I will be glad to continue.