@Hannah Could be the Gmail Cache in EM Client is a bit corrupted.
To clear the cache from all your current local Gmail Synced folders and Re-Sync the Gmail Cloud folders & email again to your local EM Client do the following.
“Right Click” on the Gmail “All Mail” folder (under your Gmail account) and then click “Properties” on the menu at the bottom. Next click the “Repair” Tab at the top right. Finally click “Repair”. You will normally then see all your Gmail folders disappear, and then re-appear after a bit and your email come back. When you click on your email again, it will take a short time to re-read each email till cached.
Next if that still doesn’t work, then you will have to completely remove your Gmail Account and Re-add the Gmail account as new as might then be a corrupted account in EM Client.
Lastly if a new Gmail account doesn’t work either, then uninstall EM Client and delete the database when asked on the uninstall and download the latest V8 client from the Em Release history page and install and setup as new - https://www.emclient.com/release-history
Note:- Before you do any of the above, click “Menu / Backup” to make a backup of your current database if you don’t have one already so you can restore if needed.