My gynecologist has me using metrogel for one night at the end of each period, which is tonight. is it safe to drink alcohol tomorrow evening?

Safe any time: The is an unusual prescription -- maybe intended to prevent bacterial vaginaosis (?). In any case, there is no concern at all about alcohol. Metronidazole, the active ingredient, is said to increase risk of nausea and vomiting with alcohol, but that's only when taking it by mouth -- and even then it's a rare problem. You don't need to avoid alcohol at all because of vaginal metrogel.