Gear 5 isn't a "gear" it's a new interpretation of his limits, mentally, physically, & spiritually. With him embracing the limitless ceiling of his fruit he's able to finally understand he can have as many forms or "gears" as he can dream.
The thing about the "gears" is (all my opinions) they're just made up. Luffy is on the brink of being both the god of liberation and destruction because he's finally realizing he has no limits. What are the gears? Limits. When he needed to push harder, a new "gear" was added