In Galaxy's left menu, when I click the Recent, it shows "Recently played by others" and lists Death Stranding, Ghost of Tsushima and Animal Crossing.

I don't have any friends in Galaxy.

Where are those game entries coming from? Why am I being shown this? Most of them are not even available on PC, let alone GOG. It's confusing to see them there.

'Recently played by others' not only refers to your GOG friends but to everyone using GOG Galaxy.
And since you can connect other platforms like Steam there are also titles shown that are not available on GOG.

Even non-PC games like ghost of Tsushima (PS4 exclusive) or Animal Crossing (Nintendo exclusive)? What's the point of this?

Well, since users can connect their Playstation and Nintendo accounts as well and these games are being played quite often, they are shown as 'recently played by others'. Surely as a PC gamer the information about recently played console games does not help you in any way. Others might find it useful. Who am I to judge... only explaining how it works^^

You shouldn't connect your console accounts if you don't wanna see non-PC games there. I also don't care what my friends on the other platforms play, but that's how Galaxy works unfortunately.

BTW those are your friends from all of your connected platforms, they don't even have to be GOG users. They will still show even if you have no friends on GOG like you said.