Depending on personal preference, it is either an expression of joy, happiness or delight or an expression of sarcasm in response to a small gain/reward etc.
This phrase describes a situation where large, gumdrop sized nipples are poking though a woman's clothing - resembling two gumdrops under the garment. This situation can be especially prevalent after swimming or under a bra-less shirt.
Another way to describe feeling hungry, feeling saliva in your mouth. No time is better to have a case of The Gumdrips than when you're starving, and can't look at food without drooling at the mouth.
'Man, that food looks so good, I've got GumDrips in my mouthalready!'