So there isn't a Google Photos Public Gallery, you say? Well, there is, Kinda. But you have to access it via Picasaweb.
This continues my rant about apparently deprecated functions from Picasaweb to Google Photos.
When you first login to Picasaweb you have to create your Google Profile. You used to have to do this to use much of Google but that has generally been eliminated. However this seems to be the key to having a public gallery of photos.
Once you create your Google Profile you are able to go to Picasaweb. You probably want to click on "View All" to see all your albums (collections?).
Now you can see all of your Google Photos albums.
To put one of these in a public gallery click on the album thumbnail and then choose "Album properties" from the "Actions" dropdown.
Under "Visibility" choose "Public on the web" and click on "Save". Realize that you just put all your photos "Public on the web."
You're halfway there.
Go back to Picasaweb and click on "My Photos" and then click on "Public Gallery".
Now copy that URL.
That's the URL of your public gallery.
Unfortunately that doesn't present in the Google Photos UI. Honestly that's a mixed blessing.
Update: If you're interested in this you'd better hurry.