the gallery don’t displaying (necessarily) files from external storage.
this is a common mistake.
the gallery displays images stored on the media store provider
you can use this method to store image file on media store provider:
public static void addImageToGallery(final String filePath, final Context context) {
ContentValues values = new ContentValues();
values.put(Images.Media.DATE_TAKEN, System.currentTimeMillis());
values.put(Images.Media.MIME_TYPE, "image/jpeg");
values.put(MediaStore.MediaColumns.DATA, filePath);
context.getContentResolver().insert(Images.Media.EXTERNAL_CONTENT_URI, values);
}