NAT64 is currently only supported on the ASR1000, ASR1001, ASR1002, CSR1000v and ISR4451 platforms.
If you want Cisco NAT64 implementation then you will need a CSR1000v image run as a VM which you may be able to integrate into GNS3.
AFAIK there isn't a demo download for the csr1000v :(
cheers,
Seb.