After purchase when you go to download directory https://codecanyon.net/downloads in codecanyon and click download you probably get some option to download like All files and documentation, Installable wordpress file only etc.
If you download All files and documentation you will get galeria.zip or galeria-main.zip file. galeria.zip is the main install-able file. If you get galeria-main.zip file then unzip it and inside you will get galeria.zip file which is the main install-able file. Do not upload galeria-main.zip file. It is not plugin file.

If you do not know how to install wordpress plugin then consider reading this article https://www.galeria.wpapplab.com/installation/