Gruntled is a word...

Gruntled is a word, since at least 1926, at least according to W.P. Webb, whoever he was.

Gruntle is a back-formation from disgruntled.

Before him - we think you have to go back to Middle English:

[dis- + gruntle, to grumble (from Middle English gruntelen, frequentative of grunten, to grunt; see grunt).]