When people reply to an email, Gmail groups responses together in conversations with the newest email on top.
A conversation breaks off into a new conversation if the subject line changes or the conversation gets to more than 100 emails.
- Open Gmail.
- In the top right, click Settings
.
- Click Settings.
- Scroll down to the "Conversation View" section.
- Select Conversation view on (messages will be grouped) or Conversation view off (messages won't be grouped).
- At the bottom of the page, click Save Changes.
<source: https://support.google.com/mail/answer/5900?co=GENIE.Platform%3DDesktop&hl=en>