The GUI doesn't work for me either, but I was able to successfully create the firewall rules with an equivalent gcloud command.
To test, I created a new VPC, new subnet and new firewall rule:
error@cloudshell:~ (strange-passage-193919)$ gcloud compute networks create ipv6test --project=strange-passage-193919 --subnet-mode=custom --mtu=1460 --bgp-routing-mode=regional
Created [https://www.googleapis.com/compute/v1/projects/strange-passage-193919/global/networks/ipv6test].
NAME SUBNET_MODE BGP_ROUTING_MODE IPV4_RANGE GATEWAY_IPV4
ipv6test CUSTOM REGIONAL
Instances on this network will not be reachable until firewall rules
are created. As an example, you can allow all internal traffic between
instances as well as SSH, RDP, and ICMP by running:
$ gcloud compute firewall-rules create <FIREWALL_NAME> --network ipv6test --allow tcp,udp,icmp --source-ranges <IP_RANGE>
$ gcloud compute firewall-rules create <FIREWALL_NAME> --network ipv6test --allow tcp:22,tcp:3389,icmp
error@cloudshell:~ (strange-passage-193919)$ gcloud compute networks subnets create ipv6test --project=strange-passage-193919 --range=10.16.0.0/20 --network=ipv6test --region=us-west2
Created [https://www.googleapis.com/compute/v1/projects/strange-passage-193919/regions/us-west2/subnetworks/ipv6test].
NAME REGION NETWORK RANGE STACK_TYPE IPV6_ACCESS_TYPE IPV6_CIDR_RANGE EXTERNAL_IPV6_CIDR_RANGE
ipv6test us-west2 ipv6test 10.16.0.0/20 IPV4_ONLY
error@cloudshell:~ (strange-passage-193919)$ gcloud compute networks subnets update ipv6test --project=strange-passage-193919 --stack-type=IPV4_IPV6 --ipv6-access-type=EXTERNAL --region=us-west2
Updated [https://www.googleapis.com/compute/v1/projects/strange-passage-193919/regions/us-west2/subnetworks/ipv6test].
error@cloudshell:~ (strange-passage-193919)$ gcloud compute --project=strange-passage-193919 firewall-rules create ruletest --direction=INGRESS --priority=1000 --network=ipv6test --action=ALLOW --rules=tcp:22 --source-ranges=2001:db8::/32
Creating firewall...⠹Created [https://www.googleapis.com/compute/v1/projects/strange-passage-193919/global/firewalls/ruletest].
Creating firewall...done.
NAME NETWORK DIRECTION PRIORITY ALLOW DENY DISABLED
ruletest ipv6test INGRESS 1000 tcp:22 False
error@cloudshell:~ (strange-passage-193919)$