I don't disagree; but the thread is looking for things post-1608 Earth time that appear in D&D, and gravity as a thing is one such.
Gravity as a
word is one such. But, it isn't like "things fall toward the Earth" is post-1608.
The Newtonian formulation of gravity did not pop fully-formed out of Newton's head alone - his formulation came after reading the thoughts of natural philosophers of the 1500s, and indeed, some as far back as the 6th century, who were arguing that Aristotle was wrong.
Though the experiment of dropping things off the Tower of Pisa somewhere around 1590 is apocryphal, the principles of it were established by Italian experimenters decades before.
So, everyone gets to decide for themselves when folks had a concept of "gravity". Aristotle knew things fell. Galilleo new that all objects fall at the same rate of speed, regardless of composition, if drag is not a factor. Newton gave us a classical mathematical formulation. Einstein gave us a relativistic formulation. And nobody has yet given us a solid quantum formulation.
Doesn't matter - in order to reverse something you have to know it exists to be reversed, and be able to identify and-or conceptualize it.
That's weird. Most people can put a car into reverse without knowing the workings of a car transmission....
You seem to be applying scientific concepts to magic which do not necessarily hold. Magic (depending on your magical tradition) does not require
understanding, so much as it requires
desired effect. Indeed, broadly, it seems to me that the more detailed the understanding of
how a thing operates, the less magical that operation will seem...
Also, the PHB is a
metagame document, so its contents do not necessarily reflect character knowledge - so the names of the spells are for the
player, not necessarily the character. Your spellcaster may not be designing a spell to "reverse gravity". They may be designing a spell to "stop falling so fast" (feather fall) or "fall upwards" (reverse gravity).