Looks like this is the problem in Install.pm:
$file->write or return Whois->error("Unable to extract file: '$path' ($GT::Tar::error)", 'WARN');
....it should be:
$file->write or return Plugins::GForum::Whois->error("Unable to extract file: '$path' ($GT::Tar::error)", 'WARN');
$file->write or return Whois->error("Unable to extract file: '$path' ($GT::Tar::error)", 'WARN');
....it should be:
$file->write or return Plugins::GForum::Whois->error("Unable to extract file: '$path' ($GT::Tar::error)", 'WARN');