There is an easy way to search for emails in GMail. Be it from a particular person or to a particular person, use the following syntax to search for mails you are looking for.
(from:(firstname.lastname@example.org) OR to:(email@example.com))
Replace the firstname.lastname@example.org with the email id in the conversation you are looking for and it’ll pull up all the results for you.
You can use various combinations in the syntax to fetch results according to your need.