Gross Misconduct Claim or Not ??

[Simon123456789]

I have a disciplinary hearing for gross misconduct ( CLAIM - striking an employee in front of other work colleagues ) I accidentially brushed the employee with my hand, who was stood behind me and had walked forward when I was showing two other staff members one of my martial art patterns I was practising. I never intentionally struck the staff member, it was a complete accident. Could they sack me?

Probably not your finest hour - showing off your martial arts moves in work time when they were paying you to work. If you can convince your employer you are useful to them and your intention was in keeping morale up for your colleagues by being a bit silly and will never do it again you may pull it back.
It was a silly mistake on your part. Just admit everything, do not try and justify, and say you are so so so sorry and want to make any reparation to your colleague that you can. You do not have much of a leg to stand on as you were caught red handed (joke - but not a good one I admit).
In this age of litigation the employer will need to be seen to be dealing with this matter so help them out and say you are totally committed to the Company and will never do it again.
Unless there are things you are not telling us then that is your only option really imho. Good luck. I expect your managers were also young once too!

Presumably if you literally brushed your colleague with your hand he/she is happy to support you as a witness?

If you are otherwise a valued and well-behaved member of staff and it was a moment of madness with no actual striking, and the colleague is willing to testify to that, then you may be ok.

If they are saying they are considering this as gross misconduct then yes they may sack you, and your job (either at the hearing, at the appeal and/or at a tribunal should it come to that) is to convince them that dismissal would be a disproportionately harsh reaction to the circumstances.