Use the very first option, Photonic → Settings → Generic Options → Generic Settings → Custom Shortcode, and fill in something like photonic there. That will reserve the photonic shortcode for Photonic and leave the default gallery shortcode alone. Then, for your Google Photos gallery you can then either replace the text that says gallery with photonic, or recreate the Google gallery, or use Photonic’s shortcode replacement capability to selectively swap out the shortcode.
I typed my response out on my phone last night, so I didn’t have access to the right terminology in some cases and my response was a little less elaborate than I would have liked. In case you are wondering how to do a shortcode replacement via Photonic directly, look under Photonic → Shortcode Replacement. This will show you all the places where you are using the gallery shortcode. You can replace individual instances with the new photonic shortcode (that you defined in Photonic → Settings → Generic Options → Generic Settings → Custom Shortcode), or you can select the ones you want to replace and perform a bulk replacement.
