Where I think Monkey D. Luffy currently scales in terms of Power-Scaling.
Conclusion: If you don't feel like reading the explanation, my conclusion is that Monkey D. Luffy would scale to Multi-Solar System level and be 225,000,000,000x the SoL (Speed of Light) in Gear 5 and 15,000,000x the SoL in base currently.
Speed:
Gear presets:
Gear 2 = 10x Multiplier (via the French translation of One Piece: Yellow - Grand Elements, where it is stated that Gear 2 multiplies base Luffy's speed and power "by ten.")
Gear 4: Bounce Man = 10x Multiplier (via Don Quixote Doflamingo's statement saying that Gear 4: Bounce-Man had amped Luffy by "tenfold," in the English translation of the officially licensed and Oda-approved Chinese translation.)
Gear 4: Snake Man = 50% amplification/1.5x multiplier (just a guess derived from Luffy directly stating that he chose to use Snake-Man because he needed something faster than Bounce-Man.)
Gear 5 = 100x Multiplier (Just read the "Egghead arc" paragraph explaining why)
Multipliers:
Long Ring Long Land Arc: Base Luffy = 100% the SoL
Thriller Bark Arc: Base Luffy = 100% the SoL
3D2Y Arc: Base Luffy = 100% the SoL | Gear 2 = 1,000x the SoL | Gear 4: Bounce-Man = 10,000x the SoL | Gear 4: Snake-Man = 15,000x the SoL |
Whole Cake Island Arc: Base Luffy = 15,000x the SoL | Gear 2 = 150,000x the SoL | Gear 4: Bounce Man = 1,500,000x the SoL | Gear 4: Snake Man = 2,250,000x the SoL |
Wano Arc: Base Luffy = 15,000,000x the SoL | Gear 2 = 150,000,000x the SoL | Gear 4: Bounce Man = 1,500,000,000x the SoL | Gear 4: Snake Man = 2,250,000,000x the SoL |
Egghead Island Arc: Base Luffy = 15,000,000x the SoL | Gear 2 = 150,000,000x the SoL | Gear 4: Bounce Man = 1,500,000,000x the SoL | Gear 4: Snake Man = 2,250,000,000x the SoL | Gear 5 Luffy = 225,000,000,000x the SoL |
Explanation:
Long Ring Long Land arc: Luffy dodges Foxy's Slow-Slow beams, with Foxy himself stating that these beams are photons and referring to them as "light" MULTIPLE times. This would mean Luffy at this point of the anime is at bare minimum 100% the SoL (100% the speed of light).
Thriller Bark arc: A heavily injured Roronoa Zoro dodges multiple of Bartholomew Kuma's pad cannons, with Kuma stating that these pad cannons are being repelled toward Zoro at light-speed, as with his Paw-Paw fruit, he can repel anything he tags with his palms at light-speed. This would further support that pre-time skip characters, such as Monkey D. Luffy, Roronoa Zoro, Vinsmoke Sanji, and other characters that scale relative to them, are faster-than-light speed at the bare minimum, as a healthy Luffy, Sanji, and Zoro are easily faster than a heavily injured Zoro.
3D2Y arc: base Byrndi World could speed blitz Monkey D. Luffy, which would put him at 10x the SoL (10 times the speed of light) via a 10x speed difference being the general norm for a speed blitz in power-scaling, Byrndi then amps himself one-hundred-fold, which would now put him at 1,000x the SoL, Gear 2 Luffy then fights on par with this amped Byrndi which would put Gear2 Luffy at 1,000x the SoL.
Whole Cake Island arc: After needing Gear 4: Snake-Man to fight on par with Katakuri, making Katakuri at bare minimum 15,000x the SoL, Luffy then runs away, comes back, and fights on par with Katakuri in his base, which would make base Luffy at bare minimum 15,000x the SoL.
Wano arc: Kaido speed blitzes Gear 4: Bounce Man Luffy, this would put Kaido at 15,000,00x the SoL Luffy. Then, Luffy comes back and fights partially on par with Kaido, which means base Luffy > previous Gear 4: Bounce-Man Luffy, which would put base Luffy at 15,000,000x the SoL.
Egghead Island arc: Borsalino speed blitzes Gear 4: Snake-Man Luffy, which would place Borsalino at 22,500,000,000x the SoL Luffy then uses Gear 5 and speed blitzes Borsalino with Star Gun; this would put Gear 5 Luffy's speed at a bare minimum of 225,000,000,000x the SoL. This would also make Gear 5 a 100x multiplier from Gear 4: Snake-Man.
Destructive Capacity:
Bartholomew Kuma explicitly states that his Paw-Paw Devil Fruit can repel anything and anyone that he tags with his palms at the SoL (Speed of Light), at the Sabaody Archipelago Kuma uses his Devil fruit on Monkey D. Luffy and repels him at the SoL, it takes Luffy 3 days to reach Amazon Lily, now in comparison to the solar system, it would only take light 14 hours to travel from one end to the other, that would mean that the distance from the Sabaody Archipelago to Amazon Lily is 5 times greater than the diameter of the Solar System, which would mean the circumference of the One Piece World is at bare minimum 5 times greater than the solar-system. So, with that being said, evidently, the One-Piece World is 5 times greater than the size of the solar system. Now, Edward Newgate (Whitebeard) was stated by Sengoku to be capable of destroying the world. Another statement comes from the One Piece Vivre Card Databook, which states verbatim that Edward "has the power to tilt the Earth." This would imply that Edward has Multi-Solar System-level destructive capacity. With the other Yonko scaling relative to Edward, they'd also scale to Multi-Solar System level.