GMail likes to present ads that are relevant to you by looking at the information in your mailbox. Fine. That is well known and just using the information you have chosen to store at Google.
However, when you use GMail to reply to a message sent by another GMail user, you will be shown ads based on the other user’s recent email activity.
That is news to me and outright scary.
I.e., do not use GMail to communicate to your doctor about an embarrassing disease because next time you write to your mother-in-law she will know. (Even if she always suspected.)