I'm not sure what might be causing her to gag and cough. No vomiting though? Have you tried cooked chicken (chopped up) and white rice to get her to eat? Is she drinking anything at all? Have you tried ice cubes? I would definetaly try to get her into a vet as soon as you can if shes not getting any fluids in her. Is she going to the bathroom at all? I know sometimes when a dogs system gets backed up they don't eat, but I'm really concerned about the not drinking part. Dehydration is just as bad for their bodies as is it for ours. Also does she have any chew toys that could have been swallowed? Again I would call your vet asap or if she seems really lathargic, dry mouth, gets pale gums, and kindda a sunken-in look in the face I might consider taking her to an ER vet to replace fluids right away and then maybe your vet can find out whats causing the problems. Good luck and I hope your pup is feeling better soon.
I don't usually get too worried about my dogs if they go off their feed for a day. Longer than that, and it's a trip to the vet. Do NOT force feed your dog. They know when they can and can't eat, and you may end up causing more trouble even with the best of intentions.
Coughing and gagging may mean that she has something stuck down in her throat that doesn't allow food to pass down to the stomach. If she hasn't been able to drink water in all this time, you need to get her to a vet. As Goldfishy mentioned above, check her gums. If they're pale pink or worse, white, that' means shock which is an emergency and you need to get moving NOW. Please don't wait and speculate any longer and get to a vet.
Call your regular vet clinic for instructions as to where to go for an emergency. Every vet clinic I've used has an outgoing message after hours that provides instructions for these situations. Please do let us know how she's doing! :-)
I would call your vet asap and speak to him - sometimes with dogs if they have a stomach upset, its advised to give them bland or minimal food for 24 hours - however if she isn't eating or drinking anything for the past 48 hours then you need to seek medical advise asap.
I know its a strange question - will she drink out of a running tap? Sometimes when my dogs won't drink from a bowl they will drink from the outside hose pipe. Have you tried to see if she would eat plain rice?