"Move to iOS" should move them all at once.
There's a method to get photos from Google Photos to your iPhone, without importing them one by one. At the top, you should find three dots at the right. (1) Tap it, a menu will appear. Choose "Select photos" (2).
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Scroll to the beginning of a day or month, tap the button there (1):
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This will automatically checkmark all of your images of that day/month. Do this for every month. Then select the Share icon at the top (2).
The next screen will show the option to save the images to Photos on your iPhone:
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That's what you have to choose, to get them to your iPhone.