Once you use “free up space” in Google Photos, how do you keep some photos for offline?
I selected some photos and tried to share them to Gmail, but it created a link to the photos in online form.
I selected the photos, and uploaded them to Google Drive.
I then made them available offline.
If you share them to Google Keep, you don't have to wait for them to be uploaded like in Google Drive.
The photos get downloaded straight from Google Photos to Google Keep.
You can use folders in Google Drive to organize the photos, and you have labels in Google Keep.
Another way to avoid uploading is to use Google Photos on desktop.
Download the photos on desktop Google Photos, then transfer them to your device through USB.
Open the photo using Google Photos, touch the three dots on the top-right and touch "Download". This will download the photo to a Download folder in Gallery. This "Download" option will not be shown if the photo already exists on your device. Also, there seems to be no way at the moment to select multiple photos for downloading.