Is "gabh" a command? I've only seen it in "excuse me" before "Gabh mo leisgeul"?
Kind of. In this instance yes, and also in the phrase Gabh mo leisgeul.
It means "take". It's just a verb.
Yes, it is an imperative form. gabh mo leisgeul literally means take my excuse, accept my excuse.
What’s the Gaelic for “I’m aff it!” ? :-)