Heard a radio DJ use this today as a sort of catch-phrase to his morning show. To make it a point to go through the day with good cheer and look for ways to compliment people, etc. Never considered that disgruntled, commonly used, had gruntle as the opposite root. And, actually, I don't think I've ever heard gruntled used on its own in my lifetime...till now. But it's a definite word:

from Merriam-Websters:

grun-tle: to put in good humor <were gruntled with a good meal and a good conversation -- W.P. Webb>

What sayest the OED about gruntled?

What thinkest my fellow wordies?

Be gruntled! I kinda like it.