First Things First: this is only an experiment, if you want to try it it's at your own risk.
For me it's working, but it's only an experiment...
Second but not least: I don't want to star here a discussion on the opportunity to use PGL or not ecc. This is not the matter.
I've understood that the normal repo of PGL it's not updated recently and not compatible with Bionic Beaver so Mint 19 refuse to install.
I've read various complaints in internet but not real solution (behind compilation, that after various years of linux life remain a great mystery to me).
So I went to a some sort of solution that is, say, an experiment, but since it seems to work I'm willing to share haw I do it.
First of all I just installed Mint 19 Cinnamon in a new partition, so Mint 18.3 Cinnamon it's intact and working.
In the 18.3 partition there is our good last version of PGL installed and working. So I've taken the 3 .deb packages that the system assemble and put in the cache folder when you install something via apt-terminal. I copied them in a folder of my 19 partition and simply installed it with double click. The package manager work on the dependencies and the installation go on right. If I'm permitted to do so, I can attach the three .deb files that I don't have in any way modified (I'm not able to). The version of this PGL its the last (2.3.1)
Second: the executable needs to be managed with root permissions but here come another surprise: Mint 19 has erased from existence also gksudo, gksu, kdesu, kdesudo and so on.
So for this experiment working you must not invoke the program from system menu (Internet) because it's not really working but instead open a terminal and put:
After you insert the root password, Peerguardian Linux opens properly, with an old interface, but seems to work well, configure the lists, ecc.
If someone understand the right syntax of the Admin:// command that seems to substitute gksudo ecc, we will be able to open the settings of PGL, insert the ADMIN:// command to open the program with Root rights but without use of the terminal.
Or... have someone an idea of how re-implement gksudo ecc in Mint 19?
Hoping to be of some help.