"Are they good eating or no?"
Lots of Cajun cooks prepare nutria. Haven't tried it but it probably tastes like bayou flavored chicken. Less greasy than raccoon which I have tried and found OK.
Should be good in gumbo. But nearly anything edible is good in gumbo as long as the roux is properly prepared. My favorite roux is darkest brown and nutty tasting and can be added to nearly any dish for extra flavor and texture.