What sa[ith] the OED about gruntled?

The most applicable definition for gruntle is: To grumble, murmur, complain

The definition given by M-W for gruntle is a modern back-formation from disgruntle assuming the more common negating meaning of dis-. The dis- in disgruntle would be the intensive rather than the negating dis-.

Gruntle is grunt plus the frequentative ending -le

See also

http://wordsmith.org/board/showflat.pl?Cat=&Board=weeklythemes&Number=107926