What you can do is add the WSA part to your Typemap file, then in your wsdl2h command specify it with the -t parameter.
i.e:
wsdl2h -o target.h -t EditedTypemap.dat target.wsdl
You can just copy the standard Typemap.dat from your gSOAP directory and edit it so it looks like the following:
wsa = <http://schemas.xmlsoap.org/ws/2004/08/addressing>
SOAP_ENV__Header =\
#import "wsa.h"\n\
struct SOAP_ENV__Header\n\
{\n\
mustUnderstand _wsa__MessageID wsa__MessageID 0;\n\
mustUnderstand _wsa__RelatesTo * wsa__RelatesTo 0;\n\
mustUnderstand _wsa__From * wsa__From 0;\n\
mustUnderstand _wsa__ReplyTo * wsa__ReplyTo 0;\n\
mustUnderstand _wsa__FaultTo * wsa__FaultTo 0;\n\
mustUnderstand _wsa__To wsa__To 0;\n\
mustUnderstand _wsa__Action wsa__Action 0;\n\
};
Then rerun wsdl2h as I mentioned above. Hope this works for you!
gSOAP is a great toolkit, just a little finnicky to get into.