I am not sure that I understand the question. GRE packets are IP packets, they use GRE IP protocol type (protocol 47) instead of TCP or UDP IP protocols so they do not have the same type of port numbers.
If you can explain a bit more clearly what you are asking perhaps we can find a better answer.
I had exactly the same requirement. Here is a configuration that works. These are only the relevant bits of the config to this question. Some of the outbound acl entries are redundant as it has permit ip any in the bottom, but you get the picture anyway.
ip access-group INBOUND in
ip access-group OUTBOUND out
ip nat inside source list NAT_LIST interface Dialer1 overload
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