In the phrase "XOR gate", this can be parsed as "XOR" (adjective) modifying "gate" (noun). Of course, this is short for "exclusive-or gate".
To be pedantic, an acronym is an abbreviation that can be pronounced as a word (laser, NASA, snafu, etc.). Not all abbreviations can be pronounced easily, and are usually pronounced as their constituent letters or letter-combinations (such as "ex or" or FBI). Since the modern trend in language is to eliminate useful distinctions, some dictionaries admit abbreviations as acronyms, to no good purpose, since we already have the word "abbreviation" and a true acronym is a special form of abbreviation.