msg-param-cumulative-months is based on the tenure of a subscription (i.e. the amount of time someone has been a subscriber), so a multi-month gift will not inflate this number. For example, if I become a new subscriber thanks to the 12-month sub gift, my cumulative months would still be 1.
This also applies to monthly anniversary notices. The message will be based on the tenure of the subscription, not the total number of months someone has been gifted.