There are 2 special considerations on iOS: your photos will also be deleted from iCloud, and the space is not released until you go in to Apple Photos and clean out the recently deleted album.

First of all, what is Google Photos “Free Up Space”
When your phone is getting full, odds are that photos/videos are a big part of the problem. If you could clear all the photos and videos off your phone, you’d have a lot more space! That’s exactly what google Photos “Free Up Space” does. After it confirms that all your photos/videos are in the Google Photos cloud, it erases them from the phone. When you use the Google Photos app, you’ll never miss them because you’re looking at the cloud and all your photos are there.
This works brilliantly on android – see video 739.GP-Free up space on Android. It also works on iPhone but with 2 concerns.
1. Free Up Space will result in deleting all iCloud Photos
If you have iCloud Photos turned on for your iPhone, it’s job is to keep the cloud and the phone in sync. So, when Google Photos erases the photos/videos from the phone, iCloud notices that and deletes them from iCloud. If that’s not what you want, you need to turn iCloud Photos OFF. You do that in Settings, Photos, iCloud Photos, then “Remove from Phone” if prompted.
You need to leave iCloud Photos off then. If you turn it back on all the photos/videos will be back on your phone and some may even be re-uploaded to Google Photos – what a mess.
2. Space is not released until you do this
On the iPhone, any photos/videos deleted from the device will be placed in an album called “Recently Deleted.” That’s like a recycle bin. They will stay there for 30 days before being deleted. If you want to free up the space right away, you need to manually empty that album.
- On your iPhone, open the native Apple Photos app (icon above)
- Tap “Albums” button at bottom
- Scroll down till you see Recently Deleted
- You should now be seeing all the deleted photos, Tap on Select at top right
- Now tap on Delete All at the bottom
If you do all this, you should now notice a difference in your iPhone Storage
- Settings
- General
- iPhone Storage