GMailThe main issue with Gmail is messages with large attachments. To find large attached files in your Gmail account you need to use a special search like this:
size:5m older_than:1yrThis example will find messages larger than 5MB that are older than one year. You can choose criteria that work for you.
This will produce a list of messages that you can review. Delete any you don't need. This moves them to the Trash where they will be deleted after 30 days. If you would like to free up that space immediately then click on "More" on the left hand side of the screen, click on Trash and at the top of the screen you will see a message "Empty Trash Now"
Google DriveFortunately, Google Drive makes finding larger files easy. Just log into your Google account and go to this link:
You will find a list of files by size. You can review these and delete them.
Once again this puts them in the Trash, which for some confusing reason in Google Drive is called the Bin.
To empty the bin click on Bin on the left hand side, then click the down arrow next to "Bin" just under the search box. This displays "Empty Bin Now". Click on this to permanently delete the bin contents.