From what I can see in the IMDB page of the movie, the writing credits are given to Dean Devlin and Roland Emmerich. Was it based (even loosely) on an existing book or is this an original script which gave birth to a book afterwards? In the latter case, is the book just a transcript of the scenario or is it worth reading even if you saw the movie?
The book was a novelization of the movie.
Also, there were several "sequel" books (NOT SG-1 related) - see linked Wiki article.
As far as "Worth reading", it's kind of a subjective thing. If you want my personal opinion - like any novelization, it's most likely far from a masterpiece of Sci-Fi and unlikely to contain anything major that would add to the movie.
However, having said that as a warning, I actually own Devlin and Emmerich's novelization of their "Independence Day" movie, and it was not terribly bad and did contain minor interesting tidbits of backstory - not the kind of book I would ever bother re-reading (this comes from someone who saw ID4 at least 10 times) but not a book I regret buying and spending couple of hours reading.
If it's a choice between reading the novelization of StarGate and reading another good Sci Fi book, I'd say go for the latter.
Stargate the movie is very, very loosely based on the book Stargate by Pauline Gedge. The book was originally published in 1982 I believe and I was just searching for it when I came across this question.
The most recent version of the book was published in 1997 by Penguin Canada, but because of the success of the Stargate TV series, and subsequent supplemental materials, the book can be monstrously hard to find if you don't know exactly what to look for.
I haven't read it since 1990, but I remember having a hard time getting into it. the ISBN is 9780140268423 if you're interested in tracking down a copy and giving it a go. It's highly philosophical and deals mostly with the immortal "star folk" rather than humans.
Star Gate, by Andre (Alice) Norton was published in 1958 and may have inspired some of the concepts embraced in the later films/series. http://www.amazon.com/Star-Gate-Andre-Norton/dp/0152787100
I Think you are talking about Gateway by Frederk pohl: Gateway (Novel)
I remember that when I read it, the back cover said that it was the inspiration for a 'highly successful movie and tv series'.