It's surprisingly simple to notarize an app using FNA. The structure of the app bundle doesn't follow macOS best practices but at the end of it you do get an app bundle that users can open up on macOS Catalina without any issues with Gatekeeper.
So here are the simple steps:
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Build your macOS .app bundle using MonoKickstart. You probably already have this and I'm not going to explain that here.
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Create a file called
mygame.entitlements
(the name really doesn't matter) with these contents:<?xml version="1.0" encoding="UTF-8"?>