How should 'Not So Fast' be read - does it counter (enemy spell) OR (ability that chooses friendly unit/gear), or does it counter (enemy spell or ability) that chooses (friendly unit or gear)?

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Read as: Not So Fast counters an enemy spell or an enemy ability, but only if that enemy spell or enemy ability chooses a friendly unit or a friendly gear. It does NOT counter your own spells/abilities, nor does it counter enemy spells/abilities that do not choose a friendly unit or gear.

Not So Fast reads as “Counter an enemy spell or ability that chooses a friendly unit or gear.” The phrase modifies both “spell” and “ability” — it only applies to enemy spells or enemy abilities that have chosen a friendly unit or a friendly gear as part of their choices/targets. Play it only when the chain item is an enemy spell or enemy ability that chose one of your units or a gear [425] [355].

Identify the chain item you want to counter and confirm whether it is an enemy spell or an enemy ability. Not So Fast specifies an enemy spell or ability, so it cannot be used to counter your own spell/ability [425]. Check whether that enemy spell or enemy ability 'chooses a friendly unit or gear' — i.e., when it was finalized/placed on the chain, did it choose one of your units or one of your gears as a choice/target? If yes, the condition is met [355] [359]. If both conditions are true (enemy AND it chose a friendly unit or gear), you may play Not So Fast as a Reaction to counter that chain item. If countered, the item does nothing and is cleared from the chain [425]. If the chosen target later becomes illegal or moves to a non-board zone before resolution, the spell/ability's instruction that depended on that target will be ignored or the target treated as invalid per normal targeting rules — that may affect whether the original spell/ability does anything when it would have resolved, but it does not retroactively change whether it was a valid target for Not So Fast when you reacted, provided the choice/target existed when the enemy item was placed on the chain and wasn't a non-board-zone change [359] [FAQ:unleashed#22].

If the enemy spell/ability 'chooses' a friendly unit or gear only as part of a cost-within-instructions or a decision made on resolution timing, the choice must exist where choices normally occur (i.e., at finalization) for Not So Fast to be legal — see the distinctions in how choices/costs are handled [355]. If the chosen object later changes to a non-board zone before any of this happens, it severs the relationship and the item may not have been choosing that object any more [359]. Also note that if the enemy item targeted your unit but that unit was then moved to a non-board zone before the enemy item was placed on the chain, it would not be the same object and would not satisfy the "chooses a friendly unit or gear" condition [359].

Not So Fast is written as a single instruction: "Counter an enemy spell or ability that chooses a friendly unit or gear." Grammatically and per normal rules parsing, the qualifying phrase "that chooses a friendly unit or gear" modifies both "enemy spell" and "ability" — i.e., the card wants an enemy chain item (either a spell or an ability) which, when placed on the chain, chose one of your units or one of your gears. This means you cannot use Not So Fast to counter your own spells/abilities, nor can you use it to counter enemy spells/abilities that do not choose one of your units or gears [425] [355].

Countering: When Not So Fast successfully resolves as a Reaction, it performs the Counter action: the targeted card or ability "does nothing and is cleared from the chain" and "is not considered to have been played" [425]. Costs already paid to initiate that enemy spell/ability remain spent [425.1.c].

Targets and choices: "Chooses a friendly unit or gear" refers to the choice/target made when that chain item was finalized (per normal choice/target timing rules). Triggered or activated abilities that include a choice about a unit or gear become chain items that contain that choice when they are placed on the chain; that choice establishes the relationship relevant to Not So Fast [355] [359]. The FAQ clarification about Alpha Strike and Repulse explains that a spell which targets an object creates a targeting relationship that persists while on the chain so long as the object remains the same on the board; the only time the relationship is severed is when the object changes to/from a non-board zone, in which case it is no longer the same game object [FAQ:unleashed#22]. That means if an enemy spell chose your unit (and the unit is still the same object on the board), Not So Fast may be used to counter it even if that chosen unit later temporarily becomes illegal in some other sense — but if the chosen object moved to a non-board zone between the enemy item being placed on the chain and your reaction, then the relationship is severed and it may not qualify [359] [FAQ:unleashed#22].

Examples: You may use Not So Fast to counter an enemy spell that targeted your unit with "Deal 3 to a unit at a battlefield." You may use it to counter an enemy ability that chooses to move your gear. You may not use it to counter an enemy spell that targeted only enemy units (it didn't choose a friendly unit/gear). You may not use it to counter your own triggered ability that chooses a friendly unit or gear because the card requires the spell or ability be "enemy" [425] [355].

In short: both the "enemy" qualifier and the "that chooses a friendly unit or gear" qualifier apply; both must be true for Not So Fast to be a legal counterplay [425] [355] [359] [FAQ:unleashed#22].