Can I share my Google Photos to Instagram using my iPhone?
From Google's support document for sharing photos and videos from Google Photos:

Open the Google Photos app.
Tap the photo, album, or movie to share.
Tap the Share button (bottom-left).

From there, to save to your device, tap Save Image on the middle row.
To upload to Instagram, tap More (bottom-right button) and Instagram should appear as an option in the second row of icons up from the bottom. (If it doesn't, scroll that row all the way to the right, then tap More, and tap the button across from Instagram to turn it on. Tap Done at the top, then tap Instagram.)