A game by nature can't come close to encompassing all the things that might happen out there or might need to be answered.
A game that doesn't have fantasy or sci-fi elements can do this. And some sci-fi games can do this too. Because in those games it really is the case that you can extrapolate from real-world scientifc knowledge should you wish to.
[Gygax]was big on not trying to simulate reality. That doesn't mean that an approximation of physics wasn't in his game.
What bit of physics, approximated, is part of AD&D as published by TSR? Not nuclear physics (because the elements are air, earth, fire and water). Not chemistry (see previous point). Not biology (many creatures, certainly sentient ones, are created, and have non-material components - minds/spirits/souls - that generate material effects, like movements of body parts when the mind wills it). What's left, besides a few remnants of common sense that were common knowledge among human beings long before the idea of physics as a science was even conceived of?
Feel free not to envision gravity when you play. Nothing is requiring you to, but the fact that you need wings to fly, things fall when you drop them, you have limited jumping ability due to the force pulling you back down, you have weight, etc., means that there is gravity in D&D
No it doesn't, because none of those things show that gravity exists! Aristotle knew all those things, but knew nothing of gravity.
Gravity does not mean "unsupported things fall to earth and can't just take off from it". Even a 3 year old knows that.
Gravity means that a force obtains between all masses, proportionate in some fashion to their product. None of the things you mention show that such a force exists.
If a D&D character could replicate the Cavendish torsion balance experiment,
that would show that gravity is part of the gameworld. But no D&D book I've ever read has discussed the result of that experiment, or whether the equipment and knowledge needed to perform it (eg wires which have a calculable constant torque) is available. And for good reason! - it's not a sci-fi game, it's a fantasy one.
It doesn't matter if Aristotle and his toga maker knew about gravity or envisioned it. It existed for them the same as it does for you. The same applies to D&D.
Well, Aristotle lived in a real world that had properties (like universal gravitation) that he was unaware of.
But I think we all agree that the D&D world is not real, and hence does not have mind-independent properties in the same way.
If Aristotle, rather than Gygax, had written the DMG very little about it would have to be different (it alludes to some technologies and some social forms that Aristotlte didn't know about). He certainly wouldn't have had to change any of the rules around falling damage, nor dragon flight, nor Gygax's discussion of flying to the moon on a winged steed.
I think that's a sufficient demonstration that nothing in Gygax's DMG assumes that
physics (as opposed to common sense - dropped objects fall, creatures without wings can't fly, etc) is part of the gameworld.
The contrast with (say) Traveller, or even Call of Cthulhu, is in this respect rather marked. Both game systems posit worlds which are chock-full of stuff that Aristotle could not even have conceived of.