Gamism does not require well-defined and agreed-upon rules mechanics. Note that I am not claiming we do not have a pretty well-defined system (in the Forge sense: how we decide what happens next in the fiction). But the mechanics, as in how precisely we figure out what happens next, are loose and partially negotiated on the fly. Rulings, not rules, kind of, but a more extreme take.
Here is the rules framwork (in Finnish, feel free to try your lack with machine translation): https://ropeblogi.wordpress.com/2020/04/15/vanhan-liiton-henkiset-saannot/
Summarizing: A character has attributes, rolled 3d6 in order, a background which tells what they can do, and then a special ability (player-chosen) or a bunch of spells (random but conform to a player-chosen idiom of magic). They have some grit (hit points) and some equipment.
The basic resolution is rolling d20 plus relevant attribute, either as a versus roll or against a static difficulty level. Characters tend to have d6 grit and a successful attack tends to deal d6 damage, and if hit points are not enough, there are rules for getting wounded and dying.
These are pretty baseline stuff and could trivially be replaced by any other basic mechanical framework without affecting gameplay much (and I have run this kind of play with different mechanical rules). The gameplay is the kind of challenge-oriented sandbox OSR play that Eero and I have sometimes discussed; neutral referee who simulates the fictional world and so on. The referee-facing rules are prepared adventure locations and NPCs, random encounter and other tables, reaction and morale rolls, etc.
My question is: what precisely makes this engineering-thought play? Is it that I often declare difficulty levels for things before rolling, or can in any case explain them afterwards? Note that the consequences are usually not declared, but they are often clear or implicitly communicated. Is it that the referee does not aim for a particular mood and genre and does not steer play (by genre I mean: comedy, tragedy, heist movie, war story; this depends much on how well the players do and what the dice give)? Something else?
I wrote a game report on Adept play: http://adeptplay.com/actual-play/terrible-mushrooms-and-loose-rules