Sometimes Gmail decides your session has expired, when you’re in the middle of writing an email.
If you’re lucky you might be able to copy the text from the draft out. If not, the most recently saved draft may or may not be up to date.
This is bad design. Why interrupt like this when you’re in the middle of something?
If you must have sessions that expire, than at least give the user a bit more time to actually finish what they’re doing — send and/or exit the draft — and then ask them to logon again.