OK. It isn't a word. But it should be. If disgruntled employees are more likely to sue their employers, you need to know how to keep them ... gruntled. Right?
For that matter, why aren't people ever described as being "heveled" or "kempt"? Why don't we ever hear of "advertent" mistakes (like the one the Chicago Black Sox made)? Shouldn't the opposite of "beautiful" be "beautiless"?
English has more words than any other language, but it seems to be missing a few. What are some nonwords you wish you had?
By the way: Merriam-Webster does call it a word, describing it as a back-formation from "disgruntle." But the much-derided dictionary seems willing to call anything a word. (For example, M-W says "supercede" is a word (it's not; it's a misspelling) and that "enormity" can mean "enormousness" (it can't).