Hi,
the problem is not (just) the plugin, first Gramps Web itself needs to be made compatible with Gramps 6. And this is currently blocked by this fix which needs to be merged and a new Gramps version released:
You can follow the current status on this page Gramps 6.0 upgrade guide - Gramps Web.
If you use the search function for “Gramps Web” and “Gramps 6.0” in the forum, you may find this thread: Gramps Web & Gramps 6.0