https://gateway-api.sigs.k8s.io/references/spec/#gateway.networking.k8s.io%2fv1beta1.HTTPRouteFilter (responseHeaderModifier)

Note, this one is currently marked experimental, but likely to be promoted soon since it's very similar to the existing requestHeaderModifier filter.

There is a conformance test for this upstream (https://github.com/kubernetes-sigs/gateway-api/blob/main/conformance/tests/httproute-response-header-modifier.go), though note that an issue was reported with it: kubernetes-sigs/gateway-api#1473