Translation:Excuse me. I do not have trousers on.
From Am Faclair Beag, online Gaelic-English dictionary— gabh /gav/, -ail 1. take 2. go 3. recite
It can mean “take, go, say, (as in say your prayers), give, recite,” and other meanings in other contexts.
However, Gabh mo leisgeul is an idiomatic phrase that means “pardon me, excuse me.” Don’t try to take it apart into individual words, just learn it as a fixed phrase.