I just downloaded gp-premium-1.9.1.zip.
WP tells me that it cannot be installed because of a missing style.css.
The second try with upload it manually ended in the same error message:
Broken Themes
The following themes are installed but incomplete.
Name Description
gp-premium Stylesheet is missing
Would it make sense to rename your button to “Premium Plugin” and call it also “GP Premium Plugin”? Here is an example why everybody expects a theme and not a plugin: