Gnosticism, both contemporary and classical, is stupidly diverse, and though there are modern organizations and clubs with a gnostic flair, and im sure one is "the largest", I dont think that any of them have any real say as to what gnostics should believe seeing as how theyre still not large enough to constitute a fraction of some of the worlds "major" religions. Gnosticism tends to be a flair combined to something else, or an influence on an organization that considers themselves christian or satanist, and thats not always the case, but that still contributes a lot to the original point. As for buddhism, its hard to say, and many gnostics ive talked to may as well be buddhist, especially when you consider the more nihlistic anti materisl sects of buddhism, but id say that the major differences on an academic level are that gnostic sects tend to identify as dualists who seek to emulate a literal or abstract god opposed to the demiurge, whereas buddhists are often non theistic entirely or dont seek to be with the gods they identify with following death.