If so, what's your opiniun on them?

Of course, I would imagine that you want to know what software I'm talking about so here's the list:

* Moonlight + Sunshine: Apparently, Moonlight allows you to connect to a computer using NVIDIA's streaming software, and Sunshine acts light that streaming software without needing an NVIDIA card. Does this work well? How's the lag? (Games of interest to look into are Stranger of Sword City Revisited, Hollow Knight, and Crimzon Clover: World Ignition, in order of more to less likely to be playable this way.) I also note that Moonlight is available for Android via F-Droid: Does this work well, and could I, say, stream the GOG version of The Quest to my phone and play it using only the touchscreen?

* Heroic Game Launcher: Supposedly, this is a game launcher that supports GOG. Does this work well? How does this compare to Lutris?

* Also, is there any open source software that allows saves to be easily shared between multiple devices, like could sharing, but without needing an external internet connection? (In other words, the saves would be shared via the local network.)

Did you check Ludusavi?

Nope, I've actually never heard of it prior to your reply.

Will take a look now.

Edit: From the github page, it looks decent, except for the OpenGL requirement (which could, for example, be an issue if a Raspberry Pi is involved).

They both have pretty silly names.

I do that with ./play.it + sshfs. While ./play.it does not include any saves/config sharing system, it moves saves into ~/.local/share/games/ if they are not already in a XDG-compliant path. Then it is easy to share this path between multiple computers using sshfs.