If it is indeed gapeworm, the valbazen will kill them, and all other worms they may have. Ysaville provided you with the correct dosage. Gapeworm will require several dosings. I would dose them at least 3 times, 10 days apart if I were you. If necessary, dose them a 4th time. Withdrawal will be 14 days after the last dosing.
If they still gape/choke after the 4th dosing (if necessary,) I would suggest you might be dealing with a respiratory disease instead of gapeworm.
If they still gape/choke after the 4th dosing (if necessary,) I would suggest you might be dealing with a respiratory disease instead of gapeworm.