The same situation is described in this topic (https://www.gavick.com/forums/john-joomla3/how-install-gkcontact-plugin-get-error-38395.html), but to be negotiated solution.
When installing the plugin, I get the message: Not found XML setup file.
Ostrzeżenie
JInstaler::Instalacja: Nie znaleziono pliku instalacyjnego XML
Other plugins have been installed without any problem.
I tried installing on a local server and hosting, every time the same message.
I downloaded the file again, but it did not help.
File Source -> https://www.gavick.com/download/joomla-3/john/gk_john_rest_files_j3-zip.raw