Answer:
Gala with a long a is the first pronunciation offered and the preferred one by the dictionary people. However, the short a or even "gah-la" are accepted, alternative pronunciations.
It's too bad it isn't more settled, because those who say, "gay-la" think "gal-ah" or "gah-la" are wrong, and visa versa.
They're all correct - but GAY la is the first, and preferred pronunciation.