To make more room for Gmail email and new photos you will have to:
1. visit https://takeout.google.com/
2. From here you can choose to deselect all as all are selected by default
3. Scroll down and check the box next to Google Photos.
4. Scroll to the bottom of the page and click “next step”.
5. Now you will be presented with a delivery method, select send the download link via email, frequency 1 export, and leave the File type & size alone.
6. Click create Export
Allow Google a few minutes to build your archive, shortly thereafter you will receive a download link for one or many files, depending on how much data you have.
Once you have downloaded the .zip archives of your photos please confirm they all look well by extracting the .zip archive data by double-clicking and following the on-screen prompts.
At this point, you can go ahead and visit https://photos.google.com to delete the photos you just downloaded a copy of from Google servers.
I hope that helps, let us know if you have any questions.