Gaelic is an adjective that means "pertaining to the Gaels". As a noun, it may refer to the group of languages spoken by the Gaels, or to any one of the languages individually. In particular, the different Celtic languages spoken in both Ireland and Scotland are very often referred to just as "Gaelic", which can cause confusion.
Literature, most generically, is any body of written works. More restrictively, literature writing is considered to be an art form, or any single writing deemed to have artistic or intellectual value, often due to deploying language in ways that differ from ordinary usage.