Learn Italian: Avessi consultato il navigatore! - If only I had consulted the GPS!

This is an independent exclamative subjunctive. When a past perfect subjunctive (congiuntivo trapassato) stands alone as an exclamation, it expresses a strong regret about an unfulfilled past action. It essentially has a built-in "If only..." meaning.

Since avessi is the form for both io and tu, could this sentence also mean "If only you had consulted the GPS!"?

Yes, absolutely! Because the conjugation for both io and tu is avessi, you have to rely on context to know who is being blamed for getting lost. You could add io or tu for clarity, but it is usually dropped in spoken Italian.

Could I add a word like se to the beginning of this sentence?

Yes. You can say Se avessi consultato il navigatore! to mean the exact same thing. Adding almeno (at least) is also very common to add emphasis to your regret, such as Avessi almeno consultato il navigatore!

Is navigatore the normal way to say "GPS" in Italian?

Yes, Italians use il navigatore (short for il navigatore satellitare) for a car's GPS or a map app on a phone. You will rarely hear them use the English acronym GPS in casual conversation.