This is part of the G5 Installation kit,
PN 010-12493-10. Runs about $60 from Garmin directly, maybe cheaper elsewhere. But also comes with the dsub connector.
I also found it surprising that at least the ring itself isn’t part of the core G5 package, as how else could you mount it? I suppose you could use the diagrams and layout the top pin hole and the bottom 3/32 screw hole exactly right, but that won’t work very well if you already have an old instrument hole. In the end, faster (and therefore cheaper) to just pay…