What would you like to be added:

Support for merging Gateways should have standard & documented mechanic.

Why this is needed:

Prior Discussions: #1248, #1246

Knative generates on demand per-service certificates using HTTP-01 challenges. There can be O(1000) Knative Services in the cluster which means we have O(1000) distinct certificates. The Gateway Resource is a contention point since is the only place to attach listeners to gateways with certificates.

The spec currently has language to indicate implemenations MAY merge Gateways resources but the mechanic isn't defined.

Secondly, given similar use-cases/needs for control planes to manage listeners it might make sense that merging Gateways is part of extended conformance.