the GalleryExtension is only for version 2.0! (see also the product description).
The plugin extends the WoltLab Gallery 2.0 largely with functions that are already included in version 2.1.
There is a new product (with video capabilities) in development, further information will be announced in the coming days.
I am most interested in the Gallery Extension for Woltlab 2.1 Gallery as I want to upload Videos to my Gallery.
Will be looking forward to when you can release the "... new product (with video capabilities) ..."!