Quoting from FreeBSD/ARM on the Raspberry Pi family
For Raspberry Pi 4 (NOT supported yet) use the image for RPI3
You might want to try the latest FreeBSD-13.0-CURRENT-arm64-aarch64-RPI3 (410MB).
No RPI4 has been reported installed yet.
FWIW, as of Linux see Support of Raspberry Pi.
Using deb files written for Jessie can be problematic with mismatched dependencies, but if libssl-1.0.0 is the only missing one it has complained about so far, there's hope.
Here is a recipe to work around this not-too-uncommon error at your own risk:
ar xv zipgateway-2.81.03-Linux-armhf.deb
mkdir -p tmp
tar xvf ../control.tar.*
sed -i 's/libssl1.0....
You were almost there with your command. After you download the package, run the following command:
python3 setup.py install
That is how you install a package (you were missing the install at the end).
If you have no internet access, you will need to solve the dependencies by hand (i.e., install ALL the packages required), so you will need to do a bit of ...