"Go!" is most certainly a complete sentence. In imperative sentences, the subject "you" is always implied.

Also, subjects do not require a predicate, they (usually) require verbs! A predicate is a word that is pointed to by an intransitive verb. Take for example,"I am a writer." Writer is the predicate, as opposed to, "I killed a writer." There, writer is the object because here the verb is transitive.

But getting back to the OP's point, single words that answer questions, such as, "Yes," "No," and "Maybe," are called pro-sentences. Think of them as pronouns, but for sentences.

In formal writing, pro-sentences are completely acceptable and it's merely a question of semantics whether you consider them full sentences.

While it is completely acceptable to capitalize the first letter of a pro-sentence and end it with a period, on the other hand, a pro-sentence is not considered formal without a traditional complete sentence preceding it.