Make the DID for one inbound route the number of Sipgate A, the other route specify the DID for Sipgate B. You dont have to specify the CID unless you know the CID of who’s calling your and wish to direct the call based on CID AND DID.
Here’s a good place to start learning:
http://wiki.freepbx.org/display/HTGS/Configuring+your+PBX