"Since the custom uploader functionality posts files not to the
Uploads folder, but to the Plugin folder, and moves them to the
Uploads folder afterwards, you should have permissions set to 755 for
the folders wp-content, plugins, gallery-plugin, upload, files.
Here is some useful information for you
You can use standard uploader on the same page of the gallery editing.
Please update the gallery after image uploading and the uploaded images will appear in the block above the custom uploader.