intake manifold gasket

Usually if the intake manifold was leaking there would be coolant on the engine or floor, So just watch the level and if it continues to drop you should have a shop check for leaks

I don't see how an intake manifold gasket could be causing you to loose coolant. More likely it's a head gasket. $$$$$$ There ares some excellen leak stop products available. Porter-Seal and Barrs Leaks are the two I recommend. Also, ZEREX anti-freeze does a pretty good job. I would not recommend Prestone Super-Sealer. Although it will stop leaks (even serious ones) your heater core can become clogged, and the heater will not work. Otherwise, get out your checkbook, you have a serious problem.