B777 Captain reported tow back to gate to correct DG load configuration error. Additionally; Captain reported no NOTOC had been provided before pushback It was determined that a NOTOC was not required due to operating under DG a regulation exception.

Narrative

After receiving salute and starting engines Ramp [Agent] came back out and gave us the stop signal. We acknowledged and they plugged back in and told us we had to be towed back in because they could not enter the ramp weights because the dangerous goods weren't properly secured. This was a surprise to us because we had received the No Dangerous Goods Planned message during preflight. During tow in we did an ACARS request and again received the No Dangerous Goods Planned message. Operations was unable to give us a satisfactory explanation and at one point said the DG (Dangerous Goods) cargo was 1;750 lbs. After a call to Dispatch and phone patch with load planning we learned that the weight was a lot less and because of the radioactivity level of the cargo a DG form was not required; even though it required specific loading by the ramp. It was confusing because ramp kept saying Dangerous Goods. We did find in the FOM where certain lower radioactive cargo does not require notification and it was confirmed by load planning that our cargo met that criteria.