There is no built-in way to download Google Photos back to your phone gallery. However, there are some workarounds you can use.
One workaround is to download the photo to your computer, then email it to yourself. Then, on your phone, open the email and save the attachment.
Another workaround is to use a third-party app like Piktures (https://play.google.com/store/apps/details?id=com.diune.pictures&hl=en). Piktures lets you download Google Photos to your phone gallery.