Gnosticenes are the human-like natives of Gnosticus IV. The only known Gnosticenes are the eight Council Members, their janitor, their Council Pages, and Princess Lindsey. It is very likely there are other, regular Gnosticenes. All these Gnosticenes (with the exception of the Princess) seem to live around the Oracle, far to the east of the Shadowlands.
In the games, only the Council Members are refered to as "Gnosticenes". However, due to the fact that they are almost the only Gnosticenes shown in the game, and that the planet itself is called Gnosticus IV, it is very likely that the whole race is called that way.
Not much is known about the Gnosticene civilization and lifestyle. There might be a Gnosticene city around the site that the Oracle is located. Princess Lindsey might be the ruler of all the Gnosticenes, or only the Gnosticenes in that city, and may have moved to the Shadowlands to help free the Council Members. Physically, they seem to be a lot like humans. The main difference would seem to be the magical powers some of them seem to possess.
It is worth noting that the word gnosis is Greek for knowledge, so the name of this planet and its inhabitants is probably based on the fact that the Oracle, a "monolith of knowledge", is in the planet.
There is a level in the game, the Pyramid of the Gnosticene Ancients, that suggests that there was an ancient Gnosticene civilization (the "Ancients") that built the Pyramids of Shadowlands. Apart from the pyramids, not much is known about them.
It is not known if the (very crude) stone houses found on several levels (Border Village, The Perilous Pit and the Isle of Tar) are Gnosticene homes, though there are no Gnosticenes found nearby. (Even though in the map both Slug Village and Hillville seem to have stone houses, there are none found inside). As the Gnosticenes are the only advanced sentient species known to live in Gnosticus IV, it is very likely that they built those houses. It may be possible that they are either very old (like the pyramids), and were abandoned years ago, or that they were recently vacated after the Shikadi increased the level of danger in the Shadowlands. It is not very probable that the Shikadi built those houses, as they weren't long enough in the Shadowlands to need houses, and as their structural style is quite different.
Other structures found in the Shadowlands, such as the Well at the entrance to the Well of Wishes, the dissapearing city of Miragia, and the Sand Yego fortress (which seems to be artificial from the map), were probably built by Gnosticenes too. However, they seem to have abandoned all these dwellings. The Wetsuit is probably of Gnosticene manufacture as well.
An interesting mystery is a huge sculpture of a Poison Slug King found under Border Village. The Poison Slugs don't seem to be advanced enough to have built it. If Gnosticenes built it, it may be possible that at one time there was some sort of tribal religion centered around Poison Slugs.