Fun linguistics fact, but gruntled as the antonym of disgruntled is a back-formation. The word disgruntled is a bit strange, in that it uses "dis-" not as a reversal prefix (such as in dissatisfied or dissimilar), but as an intensifier. The original "gruntle" was related to grunt, grunting, it was similar to "grumble", denoting the sounds an annoyed crowd might make. But this old sense of gruntle, gruntling, gruntled has not been used since the 16th century. And in the past century, people have started back-forming a new "gruntle" by analyzing "dis-gruntled" as using the more common meaning of "dis-".

A similar use of dis- as an intensifier apparently happened in "dismayed" (here from an Old French verb, esmaier , which meant to trouble, to disturb), and in "disturbed" (from Latin a word, turba, meaning turmoil). I haven't heard any one say they are "mayed" or "turbed", but people would probably see the same as "gruntled" if you used them.

Off-Topic, but do I have a story for you

https://www.ling.upenn.edu/~beatrice/humor/how-i-met-my-wife...

I’ve only heard it from Michael Scott: “Everyone here is extremely gruntled”.