English: “They gathered” means they regularly gathered there because the stone was large. “They rolled” means they used to roll it; Rashi explains that a recurring action can be expressed in either the future or past form.
On the daf / in context: Rashi reads the verbs as describing a recurring practice at the well: the flocks regularly gathered because the stone was heavy, and the shepherds regularly rolled it away.
Type of Learning
Peshat / classical peirush — not a formal named middah of the 13.
What Is Learned
Rashi resolves the verb forms by reading the scene as habitual action and emphasizes the practical reason for the gathering: the stone was large.