Trimming Message Threads When Replying/Forwarding - Gmail

If you want to trim excess messages from an email thread before replying or forwarding, Google may have hidden the excess messages by default. You will need to show the trimmed content in order to delete it.

In the body of the email when replying, click the three dots to show the trimmed content. This will expand the thread in the text box and let you select and delete past text as desired.