Depends on what you mean by "gnostic". In the sense that the outlook resembles the belief that God/divinity resides within all of us, and the kingdom of God can be realized here on earth if we come to grasp that, it's gnostic. If by "Gnostic" with a capital-G, you mean in accord with the religion and beliefs of the Gnostics, then it's really not Gnostic at all.
how come the church founded by St. Thomas the Apostle in india didn't have this book???
Because it's not really entirely clear that Thomas went to India? Or if he did, as he probably (but not definitely) did, the accounts of him there have been pretty clearly embellished by later writers? ;)
Not the answer you were looking for, I'm sure, but that's one option. Another possibility is that it is, in fact, a collection of the sayings of Jesus as told by Thomas, but since Thomas himself didn't write it down, where he was may or may not have any relation to where it was. Or perhaps it's a collection of the sayings of Jesus that got seriously distorted over the years of oral telling, such that while it started out as authentic, it became distorted into what we know today, which isn't particularly accurate at all. Or perhaps it's a 1'st century forgery, made by borrowing from the Gospels and adding other material in. The point is that nobody really knows, and there's no real factual evidence to say one way or another. The historical provenance of all of the Gospels is spotty, at best, and in that respect, Thomas isn't much different.