Gross Misconduct is the employment equivalent of Breach of the peace law. It is area that that can be wide ranging and can be manipulated to suit. Regardles of your job, all employment contracts will have this Gross Misconduct clause in it. I am in agreement with you....the minute he pleaded guilty in court of possesion (despite his misguided and naive claim that he thought it was the best thing to do, rather than go to trial)...well in my book, that would be reason enough to sack him on the grounds of Gross Misconduct.
Even of the club had acted without taking advice, i dont think there is a court in the land that would say they didnt have grounds to sack him.