may by have you a firewall that blocks the install, unable this firewall and :
look :
http://generatepress.com/knowledgebase/installing-generatepress/
if it’s not possible with this instructions :
unzip gp-premium
go in your ftp on WP-Content/Plugins and transfert yous files in this folder.
(http://codex.wordpress.org/Managing_Plugins#Manual_Plugin_Installation)
in the end reactivate your firewall
good chance to you