Even though the knowledge sought isn't esoteric and no occult practices are used, secret info. is being sought. Following what Satan is up to by reading demonology and reading and watching exorcism stuff too often attracts the interest of demons (kind of like the black ball in Lord of the Rings). That's seeking knowledge of what's beyond our world besides that which is revealed by God (or it can be for some, beyond the interest level prescribed by God) and what He has revealed through Judaism and then, through the priesthood Christ created to last till the end times.
Can following conspiracy theories (esp. regarding works of secret societies) be in the spirit of Gnosticism?
Trying to find out unknown or secret information - what you're calling a conspiracy theory, and I assume you are meaning to solve the mystery of it - is not prohibited in an of itself. Trying to use occult practices to solve it certainly is, but legitimate attempts to find an unknown secret by study, research, interviews, even digging in the ground, there's nothing wrong with that. Put another way, because Gnosticism is a heresy and happens to mean hidden or secret, that doesn't make seeking all secret or answers to unknowns evil.
You seem to be confusing this with divination. You might want to review the CCC articles 2115-2117. Those deal with divination and use of magic. The basis of it is using some form of occult practice in order to "know" things we aren't supposed to, mainly the future. You'll also note that the CCC points out that these things are rooted in a desire for personal control over others.
I was just saying "in the spirit of". We can't really do anything about it anyway, as who'll defeat the Masons and Bildebergers? All we can do is pray for them and for their plots to be defeated and not join such groups or use occult practices they use ceremoniously or in pursuit of knowledge and abilities they're not privy to--not from God, who knows more that the devils who'll use them.