Can the list ability be reacted to?

Yes — but only the [T] activated ability, not the "as you play" naming.

The List's [T]: Give a unit with the named tag -2 [S] this turn. is an activated ability. Activated abilities go on the Chain just like a spell, which creates a Closed State and gives opponents a window to respond with Reactions (Rules 377.3, 406.4). So your opponent can react to the -2 [S] before it resolves.

The "As you play this, name a tag" instruction, however, happens while playing the Gear itself — permanents finalize immediately with no Reaction window, so that part can't be responded to (Rule 9107). Timing note: the ability is still restricted to your Main Phase in an Open State unless it gains Reaction — your opponent can't fire it on your turn.