Lost iPhone 6 Plus how to retrieve all my photos in gallery ?please help me.

If you had iCloud Photo Library turned on you can see the photos by using a computer and logging in to https://www.icloud.com/ and go to the Photos app there. You can download them to a computer from there or you can turn on iCloud Photo Library on a replacement iPhone (or other Apple device or Windows computer) and they will automatically sync.

If you didn't have iCloud Photo Library on, did you have a backup to iCloud or iTunes? If so they may there and available to be restore to a new iPhone.