These are two different types of Garmin devices, both integrating navigation, coms and more.
The GNS is the oldest of the two. The GTN is the newest and provides an intuitive touch screen interface.
The most authentic simulations of these two devices are available in every simulator versions, and fully integrated in every aircraft panels, from FS9 to P3D5 and from XP9 to XP11 since 2002 (later for the GTN), except on FS2020. Both are actually using the real Garmin code under agreement and you’ll find a lot more info here: Reality XP GNS and Reality XP GTN
As for FS2020, this is a good question. I don’t believe I have a solve for your question, but related to it, here is a topic about this you might want to check out to see if you agree with:
Open up communications with Reality-XP - Self-Service / Wishlist - Microsoft Flight Simulator Forums