I always ask dealers about that stuff when I see them doing gas cards, give aways, etc and most of the time I've heard that the customer that came to the dealer to drive a car and didnt know about the offer gets positively surprised but them on the other hand a lot of people that come to the dealer to test drive a car knowing about the offer they just waste everyone's time, get the present, and leave.
Converting people that come to check a $20,000 item because they can get a $20 gas card is not easy. I sold cars myself for years, I would like to get the opinion from other real salespeople on this.
The true Internet special and not the gimmick giveaway is a well thought set up:
The proper call to action/banner in the dealer's home page, 10% of the inventory marked as a special and chanegd frecuentrly, with 2 prices (was... and now is...), and a good comment with the car perhaps even about why we are moving it.
I think that a group of goodies after you buy the car will do a lot better:
Buy a car and get: 1st oil change, gift coffee card, 1st inspection, new owner clinic, 20% car wash, etc.