"The Gunslinger" is a man in tattered black on a quest in an alternative world. By the process of passing through the gate into the other universe, he collects a crew of misfits to join him.
This is understandable. The story begins with Roland (the main protagonist of the series) in his darkest place. He has become hardened, unable to love. The story ends with him making the most heartless of decisions. "The Gunslinger puts the capital "G" in Grim.
Yet this almost unbearably bleak backdrop is essential to what the story of Roland and the Dark Tower becomes. In it we learn of what Roland is--a "Gunslinger"--and how he became one. We learn of the treachery that has set the course of Roland's destiny in motion.
Most of "The Gunslinger" comes off like a flashback wrapped inside a horrible hallucination. Bad things happen and there is no fulfilling end to this part of the tale. It understandably turns a lot of folks off to going any further with The Dark Tower.
I am here to tell you to keep on pressing on. King's story is, in my opinion, turning into one of the most epic and worthwhile tales ever written. The dryness of the Gunslinger is more than compensated for in the following books.