Thanks, launching thunar as root and opening the
.deb file from within it fixed the isssue. For future readers here are the complete steps in a simplified way:
first launch thunar as root via
pkexec thunar and enter your password when prompted.
.deb file go to properties and select
Gdebi Package Installer (even if you had changed it in normal thunar, you have to do it in the
.deb file (double-click on the file) and wait for
gdebi-gtk to read the
.deb file (the main
gdebi-gtk window will be blurred out when that is happening)
Click on the
install button and voila!
Note: Since thunar has already launched
gdebi-gtk as root, it will not ask for your password again when you click the
Again thank you very much @carmar for the help