No, conclusion.

An assumption by definition is a conclusion made prior to investigation. whether you like it or not, its a scientific conclusion, not an assumption, as it was derived from an investigation in sofar practical means allowed it.

And again, its not "because they couldn't figure it out". Its because you can only break down a phenomena so far before you reduced it to the parts we only have an axiomatic understanding of. At the end of the scientific method, you are left with the conclusion in as far we are capable of refining it. We can't conclusively demonstrate that gravity is a function of mass, we can only conclude that all our present observations show a linear relationship between mass and gravity and accept this to be the most reliable baseline upon which to base our theories relating to gravity. That is to say, our understanding that gravity is a function of mass is the most reliable and accurate model of reality we have.

And that is what science strives for; refining our model of reality; but anyone with a proper appreciation for it knows that we will never actually reach a faultless model of reality. But with almost all things, we reach the axiomatic elements of it. Things we must assume because we are incapable of modeling theories without it. These are the "laws" of science, like the laws of thermodynamics, the laws of force, the laws of communication etc etc. We stick to these laws because the theories these laws produce seem to predict reality most accurately.