Great you are using this status code, most site owners don’t bother. It’s an important HTTP status!
Sounds like you followed directions in the README. Looking through the plugin code, I can’t see how that is triggered, so I have to question their accuracy.
Can you tell if your “gone” pages are being processed by Grav at all? It seems to me that if .htaccess is followed, Grav is left out. If you leave it to Grav, you don’t have a way to know it’s 410.
I can see a way this plugin could be extended or hacked if you want to go that way. The plugin uses the hook onPageNotFound, where you could probably check against a list of “other status” URLs (obtained from plugin config rather than .htaccess). Then alter the headers appropriately and load the right page/template. Is that something you’d be comfortable hacking?