Well, first, register GW with tech-prefix. Yes, it can work without, but hey, just in case.
Next, assign this tech-prefix as default one (in GK).
Last, define in GK, which prefixes will be managed by which GW-s ("zone prefix my_gk 515* gw-priority 8 my_gw")
This should (although, I never worked with ATAs).
Regards,
Nikola