Do you really need any port forwarding if you want to setup just a gateway? The only incoming connection is via radio (the LoRa part itself), the rest is outgoing, the packet forwarder in the gateway connects to the croft instance configured.
It is not the same if you want to setup the entire backend infrastructure (i.e. croft & jolie) but those are not required if just want to plug a gateway to thethingsnetwork.org system, which would be the default for the Kickstarter gateways.
And if you want to hook up any other service to the infrastructure you will be able to do it, already now is possible (since today!) to subscribe to an MQTT topic and build on top of that without having to host the backend in your network.
Cheers!