I was also too old to watch the Smurfs when they first made it to TV, but I recall reading somewhere on the Net that the reason Gargamel was after them was that Smurfs -- or some part thereof -- were a key ingredient of the Philosopher's Stone. I don't know how canon that is -- nor have I ever really had any reason to care enough to confirm its canonicity -- but hey, there's a possible answer for you other than he's doing it solely for the evulz.

-- Bob
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My race is pacifist and does not believe in war. We kill only out of personal spite.