Grace hoped this would be her opportunity.
The meeting place chosen by Grace was in the hills of a warehouse, in an abandoned warehouse not too far from Efanso.
Her method of conveying intelligence was very unique, dispatching a summoned ghost to verbally pass on messages to designated personnel, and then immediately dismissing it.
This meant that there was no possibility of the intelligence leaking, and Gaven and his party could not relay the intelligence back to Grace; they could only head to the designated place at the appointed time.
Essentially, there was no margin for error; any mismatch in the proceedings could lead to a failure in the defection.
This was also why Gaven and his associates speculated that Grace really might be planning to defect, since as a trap, the success rate seemed overly indifferent, having prepared for a long time while there's a high chance that Gaven's side may not have received your intelligence, or might simply ignore it.