Gapeworm is VERY rare... and it is best to TEST for Gapeworm before assuming it is Gapeworm.
Take a human, regular Q-tip... have someone help you.. catch and hold the rooster.. someone can pull up on his comb and the other pull down on his wattles.. he will open his mouth (do the pulling gently and evenly).... stretch his head up a bit to lengthen his neck a bit... open his mouth.. reach as far down into his throat as you can...
Now, BE SURE to hold onto that Q-tip well.. do not drop it, do not let it slip out of your finger's grip... also....watch out for his airway... reach behind and to the side... past the airway (right behind the tongue is his windpipe, you will see it... it is to be avoided with food, water, medicine, and the Q-tip).
Swab his throat are far down as you can (this will not actually be that far.. just a couple inches)... swab around.. pull the Q-tip out... if you see worms, he has Gapeworm, if you don't see worms, he does not have Gapeworm.
But, he could have some other type of worms... have you wormed him? Wormed the flock? How old are they all? When did you worm, what did you worm with? How many times, on what schedule did you or do you worm?
Search on BYC for posts regarding "Worming with Valbazen".... by dawg53... those will be the most helpful and accurate, good info and dosage information as well.
Valbazen is safe for first time worming and kills ALL types of worms, dose must be repeated -- ten days after first... withholding not eating eggs or chickens, until the 15th day afer the second (2nd) dosing. Now, if they have tapeworm or gapeworm it will take more doses to get rid of them... and it follows the same pattern, repeat every ten days for a few -- 3, 4, etc... doses.. withholding eggs and meat until 15th day after last dose.
1/2 cc/ml per Standard chicken, given orally to each chicken one at a time... avoiding that windpipe... 1/4 cc/ml per bantam or small chicken...
Also check the chickens for parasites, mites.. Valbazen will not kill mites and Sevin Dust on chickens mite prone areas as well as a bit in a clean coop, nesting boxes, roost, floor, etc... repeat in ten days if they also have mites or lice. (under the wings, in the armpits, around the vent, above the tail where the back meets the tail).
Good luck.
Good luck.