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Instead of Google Photos app, why don't you use iCloud Photo Library ? This way all your photos are stored online and you can still view them on your iPad ? iCloud Photo Library is from Apple and In iCloud Photo Library, you have a feature that allow you to upload all the photos to iCloud and then store a low size thumbinail on Photos app (i.e on iPad), so it won't take the full space and when you view those photos, they will be viewed from iCloud directly