An example of how to use the ScreenshotManager.SaveExisting() Coroutine in Gallery Screenshot. This allows you to register an existing image (an image that already exists on phone storage) to your gallery/camera roll.
As you can see we are encoding the Texture2d (which is assigned in the editor) using EncodeToPNG and writing it to storage. Then we wait until the end of the frame before calling the SaveExisting Coroutine.
You must ensure your source texture is set to Read/Write Enabled in the Inspector:
And as always, for Android you must make sure Write Access is set to External (SDCard) in your Build Settings.