The new laws on trespass as a criminal offence only apply to residential property, as I understand it. The old gym is commercial property and so trespass is not a criminal offence. That said, it is actionable but it is only the registered owner who can take action. You could do a search at land registry against the property address to find out who owns it and you could then contact the owner direct (if the address of the owner is up to date on the land registry title...)