Gape Worm?

Any advice would be appreciated as I have no experience of Gape Worm and wondered whether I should be concerned about two of my girls.  They are eating and drinking and seem happy and heathly going about their business (laying every day)  but I have noticed that two of my girls stretch their necks occasionally and open their beaks wide.  Should I be concerned?.  They freerange in the afternoons but in an area that doesn't see much other wildlife, their mornings are spent in a pen with a bark floor.  I replace the bark once a fortnight.  Their hen house is cleaned daily and they have a straw bed in there, straw is from a local farmer who stores his hay in an open/dutch barn. 

Will do them this week to be sure

Hens also stretch their necks and gape when they move food from the crop to the stomach.
I think with gapeworm the beak is open most of the time not just occasionally because the hens airways are partly blocked.