Hello @ismailsh
You should be able to know the optional parameters that Meraki supports by looking at the BGP open message. I went ahead and configured BGP on 2 MXes and looked at the BGP OPEN message after restarting one from the dashboard and it appears to show that the MX supports and participates in Graceful Restart
Hopefully this helps!
If you found this post helpful, please give it kudos. If my answer solved your problem, click "accept as solution" so that others can benefit from it