David Grace QC, representing the Eagles, entered a guilty plea early in the two- hour hearing at Etihad Stadium.
He then went about presenting a case for leniency that included a long list of character references and a contrite Gaff giving evidence that he didn't mean to hit Brayshaw in the head.
Jeff Gleeson QC, representing the AFL, described the incident as a "historically significant event" and recommended a suspension between eight and 12 matches.
The jury of Wayne Henwood, David Neitz and Shane Wakelin handed down the lengthy suspension after 13 minutes of deliberation.
Gaff will now miss the rest of West Coast's premiership campaign and the start of the 2019 season.
How many games he misses next year will depend on how deep the Eagles go into the finals.
"First of all I just want to say that I'm so, so sorry to Andrew and the Brayshaw family for the pain that I've caused them over the past 48 hours," an emotional Gaff said as he left the hearing.
"I respect the Brayshaw boys so much and the way they go about it.
"I'm really disappointed. I own my actions, and it really hurts a lot.
"The last 48 hours have probably been the toughest couple of days of my life ... the people that I've spoken to and who have seen me in that time know the world of pain I'm in and how much I'm suffering.
"I see myself as a caring, gentle and measured person and that's why it's disappointing so much more.
"I'm disappointed for Andrew, I'm disappointed in myself, I'm disappointed I let my teammates down at an important time of the year and I'm disappointed I've let the supporters down as well.
"For the time being I'm going to surround myself with my friends and family ... those who care for me most.
"But, once again, I just want to apologise wholeheartedly to Andrew and I hope he gets better soon."