No, unless your employer breaks the law. It should all depend on the circumstances. If your employer concludes, after a proper investigation, that you have engaged in conduct that is clearly identified as gross misconduct in your employer’s disciplinary policy, this is likely to lead to your summary dismissal. However, this should not be inevitable.
For example, your employer should consider factors such as your long service and clean disciplinary record, or the fact that you offered a prompt apology, and these factors could make a less severe sanction such as a Final Warning more appropriate in your particular case.
The Acas Code of Practice on Disciplinary and Grievance Procedures emphasises that disciplinary procedures should give a clear indication of the type of conduct which will be regarded as gross misconduct and which may lead to summary dismissal.
A dismissal for gross misconduct is not normally fair unless it has been made clear to employees, in the disciplinary rules, that the conduct in question is regarded by the employer as gross misconduct. In addition, your employer must make sure employees know about the rules, and about the likely punishment for breaking them, for example by using inductions and regular training, or through posters in the workplace.