As more stuff gathers, its combined gravitation grows stronger.

What question should readers ask to determine the effect in the sentence?

How much stuff is combined?

Where does the stuff gather?

Why does the gravitation grow stronger?

What happens when more stuff gathers?

Answer: what happens when more stuff gathers?

Explanation:

cause and effect are connected, then if we want to know what the effect is, we should make a question that involves the cause, here the gathering of stuff is the cause, therefore, if we know what happens because of that cause, we will automatically will know the effect, the answer to the question "what happens when more stuff gathers?" is "it's combined gravitation grows" which is the effect