this could be a number of things ranging from an incompatability problem between Netmeeting and the gateway to firewall problems to gateway configuration problems.
1) Who is the manufacturer of the gateway?
2) Are you sure that the gateway is configured correctly? For example is the gateway setup to route the number that you are calling?
3) In advanced calling on Netmeeting did you enter the gateway IP address in the gatekeeper section at the top or in the gateway section at the bottom?
4) Do you have a network sniffer capable of decoding H.323?
5) You say that the gateway is on the internet with a fixed IP address. Is it possible for the purpose of tests to connect the gateway to a hub with the PC running Netmeeting also connected to the same hub?