There doesn't seem to be a simple way to do this, but I think this would work:
1) Pull date of last policy install to the gateway with cpstat -f policy fw, field "Policy Install Time". There doesn't seem to be a way to pull this info directly from the API that I can find; was hoping to get this from the audit logs but they don't seem to be available at all via the API. This policy install date info can be found easily on the Gateways & Servers tab of the SmartConsole GUI, so it may be somehow available through the management API although I couldn't figure out how.
2) Once you have the last policy install date, invoke the show changes from-date API call to see a list of changes since last policy install date. Note that similarly to the display on the Install Policy confirmation screen, this shows you the total number of changes since last policy install, but all those changes are not necessarily relevant to the gateway in question.
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