For a version older than 4.3.6
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 and the uploaded images will appear in the block above the custom uploader.
Please use the following instructions: