the task of science is to describe the WHAT question, not HOW question. Hence,science can only answer what happenes when there is gravity (they follow the rule of gravity)
Science attempts to answer a modifyied HOW question, as in "WHAT makes gravity", which is a modified version of "HOW is gravity made", but it won't really work out.
In the last century, i read in the book, title translates to "consequence of critical scientific theory" that the what question was "made cheaper" by high thinking school of scientists, who attempted to hit the "how" question. Indeed they were of extraordinary genius, but then, science left it's own definition and got merged with philosophy.
I myself have an undergraduate degree in astrophysics, and geology (dual program), so i guess i am lucky enough to compare the science that engages with "how" question (cosmology, sr, gr ) and the "what" question (what animal is that fossil)...
I think we need some radical exploratory discovery, to handle the manifestation question.
I know this question does not answer "how gravity is manifested", but it would be nice to have a more well formed question.
Assuming you wanted to ask "What happens when a mass is placed": The answer is, spacetime bends. The mass changes the geodesics (geodesics are the lines a non-accelerating particle would follow, a generalization of straight line, geodesic to relativity, as straightlines to first law) to a curved shape. All particles now must follow the curved geodesic. A particle that followes a curved path near a massive object, is accelerating according to 1st law, which you see as the "action of gravity"
This explaination indeed consists of holes, which are not quite unambiguously solved.