2000 Honda Odyssey Coolant over flow

My odyssey start getting low on coolant level about 200 ml per month as well as some noise from the engine. Mechanic told me I might need a water pump. After that I added a additive in the radiator, when I was pouring the additive in to my system I found some kind of gunk from the additive bottle, which I picked up (almost 90%). After 2-3 days my coolant start coming from radiator in to the reservoir and then overflow from there. That causes my radiator level go down every day. Now I empty my reservoir every day. Some people told me it is problem of head gasket. My van never overheated, no signs of water in oil or any white spews and white smoke is only when I start it up in the morning. I drained my system few times already which did not make it better.
Kindly advise me what should I do, should I go for head gasket or water pump or keep filling every day.