When you download Galeria from codecanyon you will find galeria.zip or galeria-main.zip. galeria.zip is an installable zip file. You can install it from wp-admin -> plugins -> add new option. If you get galeria-main.zip then you need to unzip it and look for galeria folder which need to zip and then can upload from wp-admin.
There are lots of video tutorial in youtube on the plugin installation. Just go to youtube and search for “wordpress plugin installation”. You also may consider the following video for referrance.