FCM uses mashup and it has its own repository so that you can manage cases from multiple pega systems (even different versions). Also I believe FCM has its own license. You probably should try to use mashup to start with.
I was actually confused between Mashup/IAC and FCM.
After long struggle i understood that Mashup is used to embed pega cases etc.. as a gadget in third party sites which need not necessarily be written in Pega. However, the case with FCM is that the other application should be in Pega.
And the dependency on gateway is only on FCM and the dependency will be reduced in the further releases.