Gear 5 works on toon physics. Luffy is not a rubberman, but a RUBBERHOSE man. He's a cartoon character now operating on something far different from what Goku could fight. Why touching anything or coming into contact with Luffy, it's warped into the properties of "Rubberhose".
But that rubberhose effect is just the base effect, the true power of Gear 5, or the Human-Human Fruit: Mythical Model: Nika, is that it turns the imagination into reality.

Simply tossing Luffy into the sun wouldn't work, Vegeta tried to do the same to Arale, a similar gag character in an episode of Super, and she circled the earth within seconds to headbutt him in the crotch.
Luffy operates on that, to a greater degree. But that is where is one of it's weaknesses, it operates on what's funny or fun. Slicing and slashing attacks can still hurt him because that isn't funny. And sufficient blunt force can still harm him despite the form acting as a buffer to blunt the damage.
But the form also combines all of luffy's abilities in the other gears in addition to his Haki techniques, like the Observation Haki. Luffy's is strong enough to give him foresight enough to dodge attacks akin to Ultra Instinct, but as for speed, well we don't know how fast he is until we see him match up with Kizaru, who's Devil Fruit lets him move to the speed of light.

The rest of the world would starve.