Serious take: all, and I mean all, Gears have or had side effects, that with time Luffy managed to remove entirely or to overcome in some way.
The G2 should have put his cardiovascular system under heavy stress with a loss of life expectancy; the G3 made Luffy like a kid for some time; the G4 made Luffy completely exhausted and/or lose his haki for minutes after.
In the scene of the G5 we literally saw Luffy getting older with naked eye, with his hair becoming white. That is not the effect of some brute force hit, but, as with all the other gears, the side effects on Luffy that was not capable of managing the g5 since it was his first time, as he was not back then with all the other Gears.
Now, Kaido was the strongest of all, strongest then every Admiral or Emperor, but even if Imu turns out to be stronger or BB manages to surpass him, I don't think we will ever see that reaction, because Luffy will have learned how to manage the G5