Is GFCI an adjective or a noun?

Nope, it’s a noun not an adjective. While all adjectives modify nouns (or noun phrases, if you prefer), not all things that modify nouns are adjectives. It takes quite a bit more than that to call a word an adjective.

Terms like GCFI receptacle, GCFI circuit breaker, GCFI device are compound nouns where each piece is itself a noun.

For the matter, so too is circuit breaker. One way that you can easily tell that a word like circuit is not an adjective because it accepts only adjectives or other nouns as modifiers, not intensifiers like very. You cannot have a ✻very circuit breaker, only an easy circuit breaker or closed circuit breaker — or here, a GCGI circuit breaker.

Another test for such things is to see what happens when you use them predicatively instead of attributively. That means using the thing you’re checking whether is an adjective after an inflection of be as with ✻The breaker is circuit, which clearly doesn’t work.

A third test is to see whether the word is gradable. You can’t have a ✻more circuit breaker or a ✻circuiter breaker. Same with your GCFI: it’s not possible for one circuit breaker to be any “more GCFI” or “GCFIer” than another.

So it’s a noun.