there will be sharing of fluid between the transfer case and trans. the gasket is only to keep fluid from leaking out of the tail section and transfer case. the transfer case can accept fluid but not much as the fill port between the TC and trans is quite high at the top. plus the area of the parking gear is not filled with atf. they take the same fluid the trans and TC do.
now if its filling the transfer case then you have other problems and need to examine the seal around the input shaft of the TC.