Because I have been using Gmail and Google Drive forever, my 15 GB is finally filling up to a level that I had to delete useless email and files from Google Drive and Google Photos.
Here is a useful article that provides scripts that will help you clean up your Gmail most efficiently.
As an alternative, many colleges have agreements with Google that provide you with unlimited file storage. I had heard that this was going away, but it looks as though I still have unlimited storage with my UCLA, El Camino and SMC accounts. I had also thought that this would not work, but I figured out how to set my Google Photos account to my UCLA Google account so that all my photos are automatically backed up from my phone with unlimited storage.