It won't necessarily be important to you if you're not collecting, but the original version was when he was young and well before he finished the rest of The Dark Tower series. There is a revised version that he put out later after finishing the series where he corrects for some inconsistencies between that first book and things that occur in the following books of the series. Better continuity and more detail with quite a bit of new text added.

Of all the books he's written, the original version of The Gunslinger is not that great. The revised version is better, especially if you plan to continue through The Dark Tower series. It's definitely important to start with, but look for that easier-to-read updated version unless you're trying to collect the original, too.