Access rule priority change from the Global/Group level in GMS is not supported. This is by design and this option is only available per individual firewall changes. Images below show expected behavior.
(Firewall level) Priority setting is available.
(Global/Group level) Priority setting will be missing
If a manual rule priority is required, it will need to be set on the firewall level and pushed using reverse inheritance to the other firewalls in the group.
Related articles below on how to push group settings to multiple firewalls within GMS
Before making global or group level configuration changes and firmware upgrades, a GMS administrator should apply the change at a unit level, and then test it with a group node that has two to three firewalls. If both tests lead to expected results without causing any side effects or bringing down management access to the remotely managed units, the GMS administrator can push the changes to a larger group or global level node in GMS.