Gizzard Worms attack the musculature of the chickens gizzard and can cause illness, and death in severe cases.
Appearance:
Red
Length
½ to 1 inch* long, worms that infect 
the musculature of the gizzard.
* Length varies with age and species.
Reference: R.D. Jacobs, J.A. Hogsette, and G.D. Butcher. http://edis.ifas.ufl.edu/ps012
Symptoms of Gizzard worms in chickens are:

• Slow growth
• Weight loss
• Little appetite
• Miserable appearance
• Ruffled feathers
• Diarrhoea
• Less egg production 

Treatments for Gizzard worms in chickens are:
Gapeworms and Gizzard worms can be treated with one of the following treatments:

• Albendazole-Administered orally, ¼ cc (mL) per bantam, ½ cc (mL) per large breed. Repeated in 2 weeks.
• Fenbendazole (liquid)-3 cc (mL)/gal in water for 3 days. Repeat in 3 weeks.
• Fenbendazole (paste)-Pea-size dose/bird, administered orally. Repeat in 10 days and again in 3 weeks.
• Fenbendazole (powder)-1 oz dissolved in 1 cup (240 mL) of water, mixed with 15-20 lb (3-4 g/kg feed) for 1 day
• Ivermectin (drench or injectable)-Administered orally - 1/4 cc (mL) per large breed chicken, 6-7 drops (0.1 cc) per bantam. Administered in drinking water - 4 cc (mL)/gal of water, for 2 days.
• Levamisole (drench)-Administered orally - 1/4 cc (mL)/lb body weight. Administered in drinking water - 10 cc (mL)/gal for 1 day. Repeat in 7 days, and repeat again in 7 days.
• Levamisole (injectable)-Injected under the skin, 1/4 cc (mL)/s lb body weight (25 mg/kg)" (G. Damerow, Poutry DVM).

Reference.Worm parasites in poultry. Queensland government..(Aug. 27, 2012). http://www.daff.qld.gov.au/animal-industries/poultry/diseases-and-disorders/worm-parasites