Erased iPhone-Is it Gone forever?

When something exists as a single copy, and you erase that single copy, there is nowhere to recover data from.

So unless you’re using iCloud photo already, or transfer your photos regularly to a computer or online archival storage (Dropbox, google) they will be gone when you erase the phone since that is the only place on the planet they are currently stored.

Theoretically the data isn't erased immediately, but it will be the way that flash memory is used. Before writing, blocks have to be erased because flash memory can't be randomly overwritten.

That being said, iPhones encrypt all data and if it's erased the encryption key is thrown away. There is no practical way of recovering anything after an erase.

If the data wasn't backed up or synced to the cloud, it's gone.

There is no getting it back.