Jonas argued he had intended to hit Gaff in the back or shoulder having "braced for contact" after making a late decision not to attempt a spoil, resulting in what he described as "a clumsy mess".
But the jury, consisting of former players Hamish McIntosh, Shane Wakelin and Emmette Dunne, disagreed with Jonas's assertion, finding he deliberately struck Gaff.
By contesting the grading, Jonas forfeited the right to an early guilty plea and was stung with an even lengthier ban. AFL counsel requested Jonas be suspended for seven games, while Jonas's counsel argued that would be "extreme", adding "this is no Barry Hall punch".
The jury somewhat agreed, and settled on six weeks.
It's a fair outcome," Jonas said after the hearing was concluded.
"I'll have to rethink the way I approach the contest at times. I'll use the break as a good chance to get my fitness topped up."
The incident came late in the third quarter of Saturday's match at Adelaide Oval, and saw Gaff stretchered from the field after being knocked unconscious.
Gaff missed the rest of the match, and a West Coast medical report said he was unconscious for two minutes and is likely to miss one match as he recovers from concussion and a "moderate severity neck injury".
All other sanctions from the weekend were accepted, including a number of fines from the ensuing melee after the Jonas hit, and a two-week ban for Western Bulldogs' Lin Jong for a high bump on GWS's Matt Buntine.