Welcome to Android Central! If all of the photos are locally stored on your iPhone, then I suggest transferring them to your computer via iTunes (or using a file manager to drag and drop them to the computer--can that be done these days on an iPhone?). Then
upload the photos to your Google Photos in the cloud, which has two benefits: (1) the photos are safe in the cloud, without risk of data loss, and (2) you can access them from any device using the Google Photos app. If you want a particular photo saved to your phone, you can select it in the Google Photos app and download it.
If you have lots of photos and only the basic 15 GB of Google Drive storage, then make sure you upload the photos in "High Quality" resolution, rather than "Original." Google Photos allows you to store
unlimited photos in High Quality, which means photos up to 16 MP in resolution.
https://support.google.com/photos/answer/6220791?hl=en&ref_topic=6156061
In general, you should avoid keeping your photos stored only on your phone, because mobile memory (and especially SD cards) can fail, and phones can get crushed or flushed down the toilet.