Hi there,
Looking at Gravy's page, this requires you to give your players permission for graves to spawn with a permission plugin.
How?
If you are using Permissions EX, run /pex group default add gravy.grave
If you are using LuckPerms, run /lp group default permission add gravy.grave
If you are using GroupManager, run /mangaddp default gravy.grave
If you have any issues, please let me know.
Graves looks like a neat plugin, just wanted to say that Gravy is not broken at this time.