Gaelic vs Celtic

Gaelic and Celtic both are nouns.

Word	Noun	Adjective	Verb	Adverb
Gaelic	Yes	Yes	No	No
Celtic	Yes	Yes	No	No

As adjectives, Gaelic and Celtic are synonyms defined as:

As nouns, Celtic is a hypernym of Gaelic; that is, Celtic is a word with a broader meaning than Gaelic:

Gaelic: any of several related languages of the Celts in Ireland and Scotland
Celtic: a branch of the Indo-European languages that (judging from inscriptions and place names) was spread widely over Europe in the pre-Christian era

Other hypernyms of Gaelic include Celtic language.

Gaelic (noun)	Celtic (noun)
any of several related languages of the Celts in Ireland and Scotland	a branch of the Indo-European languages that (judging from inscriptions and place names) was spread widely over Europe in the pre-Christian era

Gaelic (adjective)	Celtic (adjective)
relating to or characteristic of the Celts	relating to or characteristic of the Celts