Like "either", this one has been debated since the dawn of time, but we're reliably informed that there is a correct answer, and we're going to share it with you. We figure it's the least we can do - if you're going to a big, fancy ball like a gala, we guess you're going to want to know the right way to say the word when you're thanking whoever invited you.
This word is a curious one because it involves two different uses of the 'a' sound within the same word. Whilst it may look like it should be pronounced "gar-lar" or "gah-lah", neither is correct, because the vowel mysteriously changes it. The right way of saying it is "gay-lah". Don't worry, this one definitely surprised us, too, so don't feel too bad about it!