Can Luffy Use Full Body Haki. Also Luffy has mastered Gear 2 and Gear 3 post timeskip. With Vergo not only did we see Full Body Haki for the first time.
18 characters including Luffy can use Observation Haki up until the recent chapter. Gear Two Viz Simulcast Fighting. With Vergo not only did we see Full Body Haki for the first time.
Luffy had fatigue because of him putting 100 Shadows into his body where a human can only take 2.
Luffy in Gear 4 has been using his haki to manipulate his rubber body so I think Full Body Haki works as a future upgrade for Luffy in a potential Gear 5 scenario but probably only after hes mastered awakening. Its only used to protect vulnerable parts of ones body to heighten defence. He can use Gear 2 in one part of his body not all of it like Pre TS making the strain less on his body. With it he is capable of sensing the presence emotion and intent of others greatly aiding his ability to dodge attacks such as bullets.