Hi Tara,
I just added a filter to the next version of Blog/GP Premium so it’s possible to change this.
add_filter( 'generate_blog_masonry_metabox','generate_add_blog_masonry_metabox' );
function generate_add_blog_masonry_metabox()
{
return array( 'post','post-type' );
}
This won’t work until GP Premium 1.2.78 is released.
If you want a head start on it you can email me at support@generatepress.com and I’ll send you the latest version.
Thanks!