As an employer, if we offered a benefit (such as a gym membership) to an employee in the form of an onboarding email can we withdraw the benefit? (It was not in the offer letter)

You can likely withdraw the gym membership benefit without exposing yourself to a claim for constructive dismissal. However, we would recommend that you consult with one our lawyers to review the terms of the offer letter and onboarding email before making any substantial changes to an employee's terms of of employment.