Will there be any downtime if i update Azure Application Gateway (for eg, disabling a few rules on the gateway) . We have configured 2 instances of Azure Application Gateway
Modifying you gateway will only result in downtime if you make a change that doesnt work, so i would always make sure to have this in a planned service time frame.
are you using two gateways for the same application ? if so you could just use 1 gateway resource with multiple instances, this is also required to have the 99.95% SLA