Gaelic is the Celtic language spoken in the Highlands of Scotland. There is a related language sometimes called Irish Gaelic, more usually Irish or Erse, which is spoken in Ireland. It is an official language of the Republic of Ireland. Nowadays, in both Ireland and the Scottish Highlands, English is more usually spoken by nearly everyone, although in the past Gaelic was the only language spoken by most of the natives.
The most usual pronunciation in England is to rhyme the first syllable with 'gale', IPA: /ˈgeɪ lɪk/. Many Scots, particularly those who speak it, or pretend that they do, pronounce it similarly to 'garlic', IPA: /ˈgɑː lɪk/. (More accurately, this is IPA: /ˈgæ lɪk/, which is intended to sound like the Gaelic pronunciation.)