Hate to break it to you all, but gdebi and dpkg can be used to install propietary software also...
The issue is not whether a program can be used to install proprietary software. The issue is whether a program comes configured by default to "steer users towards obtaining any nonfree information for practical use, or encourage them to do so".
The Free System Distribution Guidelines at https://www.gnu.org/distros/free-system-distribution-guidelines.html make this distinction:
A free system distribution must not steer users towards obtaining any nonfree information for practical use, or encourage them to do so. The system should have no repositories for nonfree software and no specific recipes for installation of particular nonfree programs. Nor should the distribution refer to third-party repositories that are not committed to only including free software; even if they only have free software today, that may not be true tomorrow. Programs in the system should not suggest installing nonfree plugins, documentation, and so on.
enduz...@riseup.net reported that Snap was configured by default to install nonfree information for practical use, such as Skype and Spotify.