When you see a gerund (a "verb-ing" acting as a noun), think of substituting the phrase "the act of verb-ing."

For example, "studying is hard work" = "the act of studying is hard work." It's assumed that "studying" refers to the entire concept of studying in the abstract, and not to one particular instance of studying.

For this reason, it doesn't make sense to use a gerund with a possessive. If we say "the scientists' assuming," and we try to swap in "the scientists' act of assuming," it doesn't make any sense. Since we're talking about one particular instance of assuming, and not the idea of "assuming" in the abstract, we want to say "the scientists' assumption."

Does that help?

Here's more info about the use of -ING generally: https://www.beatthegmat.com/ing-form-t269857.html#694856