I am very sorry to hear about your pup Tyson's loss of appetite , poor drinking, eating grass and gagging.
Dogs with nausea or gastroesophageal acid reflux often eat grass or foreign material to make themselves vomit. They will often swallow repeatedly and lick things trying to soothe their nausea and acid burn. If they can rid themselves of whatever is irritating their stomach that way then all is good.
But when they continue to be uncomfortable then we need to stop them from eating any more grass. It becomes a vicious cycle where the more they eat the more their stomach acid refluxes and burns their esophagus and the worse they feel.
Possible causes of nausea and loss of appetite causing grass eating include a sudden change in diet, dietary sensitivities or allergies, or eating things that they should not like too many fatty table scraps or garbage, bones etc. Addison's which is a poorly functioning adrenal gland is another possibility for waxing and waning vomiting and nausea. Since she's an older pup metabolic organ failures (kidney or liver disease), pancreatitis, inflammatory bowel disease or even infiltrative cancers are possible causes too.
Has she eaten anything she should not have recently (toy pieces, bones, garbage)?
Any changes in food or treats?
You can give her an acid reducer to try and settle her stomach. Either:
1) Pepcid-ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one half of a 20mg tablet per 20 to 40 pounds of body weight every 24 hours.
These will reduce stomach acid and should help settle her stomach. These can be used long term if necessary as they are very safe.
I'd also pick up her food and water for now. A couple hours after the acid reducer is given you can offer small amounts of water or ice cubes to lick.
A few hours later (or tomorrow morning) if she seems to feel better offer a bland diet mix of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken) and 2/3 boiled white rice. Give small meals several times a day. Feed the bland diet for several days, then start mixing in her regular diet and slowly convert her back.
If her nausea continues then she may need injectable medication from her veterinarian to get her stomach upset under control.
So watch her for continued lack of appetite even with the acid reducers, blood in her stool or vomit or a fever (more than 103.5F rectally), a tense painful belly or lack of appetite after her food fast. If any of those occur it is time to seek hands on veterinary care. It would also be a good idea to have blood tests done as well to make sure there isn't an underlying infection or metabolic problem like organ failure, pancreatitis or Addison's disease present.
Please let me know if you have any further questions.