Yes, this is a gimmick to get people to buy more coffee because every day now is a holiday for something obscure. Marketing rules the world, and if companies know they can sell more of their product by giving a few free samples away, they will do it every time.
National Coffee Day might not have been created with only this in mind, but coffee shops who give away free coffee on National Coffee Day are trying to get customers to come back and buy more coffee later. They think that if you like their coffee this time, you will come back later.
And I think that it is the only reason why they are doing that. Starbucks and McDonalds are not handing out free coffee. But before you complain, they will make a charitable contribution for every cup they sell on Thursday. Starbucks is donating coffee trees, Mickey Ds is giving to Ronald McDonald House.
Besides, does Starbucks really need to give out free coffee? They will still have a long line.
No, I think that it is not possible to lead people in drinking more coffee, since they are already drinking coffee as much as they can. Well, maybe not Englishmen, since they like tea more than coffee. Somehow, coffee has become the universal drink for people all around the world. I have never heard of a place in the world where coffee is not used as a refreshing drink.