While yes technically you can disable public IPv4 address on either AWS or GCP, how you can use IPv6 is quite different.
GCP unfortunately does not assign IPv6 to instances. You need some type of proxy to terminate v6 in front of your v4 only instance.
AWS can assign v6 addresses direct to instances for use. Even though users often do not connect to a single instance, it still helps to have a sane address plan, plus not waste v4 addresses on management. You still get a RFC1918 address, although that of course that isn't routable on the Internet.
Whether you can is a different question from whether you should. Make a choice in whether v6 only makes sense for your organization's users. Think about what happens if they cannot connect.
Where you control the user's network, sure it is possible. Several ways to ensure IPv6 connectivity: enabled campus networks, VPNs, cellular data plans, 6in4 tunnels.
For general access from anywhere on the Internet, probably not. Still lots of v4 only out there.