Is your feature request related to a problem? Please describe.
The Gravestone Mod is intended to keep your items safe. if it can be just deleted by the dragon, your items are not safe (even if they just pop out of the gravestone, that has a 5 minute timer)

Describe the solution you'd like
Quite simple, add gravestone:gravestone to the block tags of minecraft:dragon_immune.

Describe alternatives you've considered
I have made a datapack that fixes this problem by adding it to the minecraft:dragon_immune tag

Gravestones are indestructible without any tags. But yeah, for consistency and compatibility with other mods, I will add it.