Every watch sold on the grey market was once sold at an AD, this is (at least at 99 out of 100 brands) the only way to get such a watch. This means in 99 out of 100 cases the grey market dealer also bought the watch from an AD. If the grey market dealer is reputable he will insist on getting the guarantee papers filled out and stamped correctly. In this moment the manufacturers guarantee starts, no matter if the owner changes afterwards (at least if there is no owners name written in the papers). A completly different thing is that you often get strange looks when you tell an official service center that you bought the watch on the grey market, the manufacturers simply don't like the fact that a third person did earn money through the watch, but hey: that isn't your problem.
So what is a grey market dealer: Someone who has a good relationship to one or more ADs, he buys large amounts of merchandise there every year and through this gets better prices than me and Bobby McGee. He can resell the watches at a lower price than the AD because he usually doesn't need to pay for a 5th Ave store, a workshop, employees, advertising etc..
The decision whether you buy on the grey market or from an AD is your own. I'm more the AD guy, because I'm not after the last penny, though most of my friends buy on the grey market, even 20000$ watches. I'm OK with my way, they're fine with their way, you'll have to find yours.