hi an employee of mine who wants to leave work early and is responsible for locking up  has locked the fire exit while people were in the building can this be classed as gross misconduct

It is certainly dangerous behaviour; to start with you should explain the importance of the fire exit and give the person a written instruction. With regards to disciplinary progression you need to speak with employment lawyers.

Harry

Check with your employment lawyer, I'd be surprised if it wasn't, especially as the action could be considered a breach of Health & Safety and Fire Regulations, which are criminal offences.