This is an important observation. If the TOC find that you are 'guilty' of gross misconduct, dismissal is only one of the range of responses available to them, so you may have resigned unnecessarily; a warning, written warning, or a final written warning might also be appropriate in the circumstances - and remember even if proven, the hearing must hear and consider mitigation, including your previous employment record which you would put to them (hopefully as an exemplary employee). If it does come to the worst, your disciplinary policies allow for an appeal, and with that, I would hope that the union would escalate the matter in support of you, including to their own legal advisers, who in the very worst case would advise in the case you took the matter to an employment tribunal.
Looking at the attachments, it seems to me that the investigation / fact-find's conclusion that this may be a case of gross misconduct, appear rather thin. Is it an assumption that you have taken no responsibly or checked that the pass is being used correctly, or is it something you have admitted? And how are you expected to check, as a matter of interest? Have an annual review with your wife and get her to sign a declaration?
It may be of no comfort, but this is an interesting issue. Whilst in the course of normal relationships, partners will understand their rights and responsibilities in relation to benefits of this kind, not all relationship run smoothly and not all partners are honest. I do ask therefore, in this day and age of equality, what is the extent to which a husband can be 'legally' or otherwise, held responsible for the improper actions of his spose, unless of course you were party to the action? If TOC's need an assurance that passes are being properly used, perhaps THEY should obtain signed written confirmation from the beneficiaries, each year.
Good luck.