It's been a while but the rangers in tournament that i remember of don't interrupt casters on reaction on normal basis, they do when chance show itself like when they need to interrupt a critical skill that showed itself, but when they are assigned to interrupt lock a target and shut him down, they interrupt them on 2 things that i often see... either daze, or quickshot crit, the second build being utilized with assassin main and not actually ranger main usually, while the first one i see interspersed with poison/interrupt ranger.
on either case it's not so much they fire upon seeing the cast sign, rather they fire as many arrows as possible on the target with high chance of interrupting the target on each arrow.
i can't remember if they used choking gas for the builds, it's been a while but as far as i remember of, when my guild mate ranger or assassin/ranger is assigned a caster to shutdown, he doesn't even fire on him on reaction.. he just CONSTANTLY firing on him and almost nothing not instant ever come out while under his lockdown until he has to recharge his preparation.
effectively almost every single arrow he fires will interrupt whatever his target is doing, spell cast, standard attack even, he just fires them as fast as possible and uses his interrupt attack when there's something he absolutely must interrupt while his normal interrupt prep or passive is on c/d or otherwise unavailable.
i can't remember where the daze source comes from, i do remember it's not entirely from the ranger, our front line meleer's iirc inflicts them as well when necessary... but my memory might not be reliable there.
for mesmer i am pretty sure i see migraine in the build often.