There's a tag for gears level and in the original post it looks like everyone in the comments was confused even then what gears-level meant, and in particular there were a lot of non-overlapping definitions given. In particular, the author, Valentine, also expresses confusion.
The definition given, however, is:
1. Does the model pay rent? If it does, and if it were falsified, how much (and how precisely) could you infer other things from the falsification?
2. How incoherent is it to imagine that the model is accurate but that a given variable could be different?
3. If you knew the model were accurate but you were to forget the value of one variable, could you rederive it?
I'm not convinced that how people have been using it in more recent posts though. I think the one upside is that "gears-level" is probably easier to teach than "reductionist" but contingent on someone knowing the word "reductionism" it is clearly simpler to just use that word. In the history of the tag, there was also previously "See also: Reductionism" with a link.
In the original post, I think Valentine was trying to get at something complex/not fully encapsulated by an existing word or short phrase, but it's not clear to me that it was well communicated to others. I would be down for tabooing "gears-level" as a (general) term on lesswrong. I can't think of an instance after the original where someone used the term "gears-level" to not mean something more specific, like "mechanistic" or "reductionist."
That said, given I don't think I really understand what was meant by "gears-level' in the original, when there are suitable replacements, I would ideally like to hear from someone who thinks they do. In particular, like Valentine or brook. If there were no objections maybe clean-up the tag by removing it and/or linking to other related terms.