The gerund in French has several uses. What your question alludes to seems to address its use in expressing a timeline.
Elle est tombée en parlant. -- She fell while talking.Il lisait son journal en mangeant le dîner. -- He used to read his newspaper while eating dinner.
Clearly the gerund is used when expressing simultaneity of two actions under the condition that the subject is the same.
Marie m'a parlé en me regardant. -- Marie talked to me while looking at me.Marie m'a parlé pendant qu'il me regardait. -- Marie talked to me while he looked at me.
In the first sentence the subject (Marie) is the same and the two actions are simultaneous, hence the gerund. In the second example there are two subjects involved (Marie and il), so the gerund wouldn't work. Therefore one solution is to use "pendant que" and convey the temporal relationship of the two events using passé composé and l'imparfait.
I hope that helps a bit.
-- Chris (not a native speaker).
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