Much as Chris says.
Gross Misconduct is a fundamental breach of the contract between employer and employee such that the contract is terminated without notice. So basically you leave pretty much the same day. Listen to what they say, take a friend with you to write notes on what is said (looks efficient and might need the info later on), prepare beforehand what your case is for remaining in your job. Having listened to what they say, when they ask you for your side of the story, unless you are comfortably with what to say immediately, ask for an adjournment for a few minutes to work out what to say. Mitigate as best you can, ask that they consider a Final Written Warning rather than dismissal for gross misconduct. This shows you understand how the employment process works. If they won't 'commute' to FWW, contemplate using the appeal process - there should be one.
From the employers' point of view, if they have pretty much decided they want to get rid of you, it may not change their decision, but at the back of their mind, they must always think that you might bring a case for unfair dismissal. They will want therefore to follow a clear process.