Science at its core aims to define things: hypotheses are proposed and proven/disproven in search of pinning down clearly defined rules and patterns.

Is gravity just an assumption that differs for each of us?

If you named it vticarg instead and defined the word "gravity" as loud noises, that would be fine scientifically.

Indeed you will find "gravity" defined different ways in different papers. Sometimes it's Newtonian, sometimes relativity, sometimes MOND.