I'd like to be able to highlight portions of text in emails I receive from other people, in Gmail. When I reopen those emails (often large), I'd like to be able to see what portions of text I had previously highlighted.
Not within the Gmail web client. I would recommend creating a Google Doc, pasting the contents of the message into it and marking it up there.
You could then take the URL of the Doc and put it in a reply to that Gmail thread addressed only to you as a quick way of referencing it in Gmail.
If a group of people are collaborating on this content, using a Google Doc instead of an email thread may be a faster, more efficient way of getting the work done.
Not with Gmail all by itself. You can't edit messages once sent/received. At best, you can forward the email message to yourself with the text highlighting you want.
Otherwise, you'll need to look for a third-party solution.
Just found a way of highlighting text in my received emails (Gmail) by using Highlighter + Notes, a Firefox add-on (there might be versions to other browsers, but I'm not sure).