G2 isn't a "change on a molecular level" it is pumping blood faster around his body, to a slightly more enhanced version than is possible through soru, it's more akin to a technique, like soru is a technique, because it is how his body uniquely responds to the soru stimulus.
G3 isn't a change on a molecular level either, it is the bones of a limb getting inflated with air, often for a single attack at a time.
Is Kid building a mechanical arm to punch with not his base?
Now this distinction isn't black and white, and you could argue that they are modes rather than techniques and try to make some artificial distinction why Luffy shouldn't be able to use x, y, z techniques in some hypothetical battledome .
The purpose of such a powerscaling discussion is usually to determine effort and stamina involved usually, and g2/g3 are not a significant stamina or effort for current Luffy, so it's a bit mute, short of Luffy losing his fruit power entirely through seastone/Blackbeard, he will have access to g2/g3.
The same is not true for Zoro and 3 swords, as one sword could be destroyed or lost as we have seen, the same wouldn't be true for king of hell mode/g4 because it has haki drawback, the same wouldn't be true for Kid and his giant mecha because that takes an unusual amount of additional metal preptime.