giving up sealed information to collaborators
remember that mueller and co., were able to show manafort was lying after he initially decided to "cooperate" with respect to his contacts with people related to the investigation. if he did want to get a message out to someone this would be one way.
he may be signalling to people that its gates who has the info to bury them (since presumably gates would have been entrusted with details such as manafort's trips/agenda and the polling data given to kilimnik).