The duo will reunite with their former Eaglehawk coach Derrick Filo, who has taken the coaching reins at Mount Pleasant in the Heathcote District Football League.
Robins - one of the best centre half-forwards in not only Bendigo, but country Victoria - will be an assistant coach.
"It was one of the hardest decisions I've made to leave Eaglehawk considering we were on the verge of a bit of success this year," Robins said on Friday.
"But I've been chased by Mounts for a couple of years and the timing is right with Derrick out there coaching now.
"I was assistant coach to Derrick for a couple of years at Eaglehawk and really enjoyed my time under him."Gretgrix has been a dominant force in the goalsquare for Eaglehawk.
In seven seasons with the Hawks, Gretgrix kicked 474 goals - including 71 this year- and was the club's leading goalkicker six times.
"I'm really excited about heading to Mounts and playing under Derrick again," Gretgrix said.
"He got me to Eaglehawk and I've always been keen to play under him again. I like his style and the way he goes about it.
"It has been a big decision to leave Eaglehawk, but I'm looking forward to the challenge."
Filo said his recruiting was purely based around quality.
"We've got a good group of young players at the club who we want to keep some senior spots open for, so we've focussed on getting in quality, rather than quantity," Filo said.
"Kain and Matt are certainly two quality players that we're rapt to have on board."
As for luring two star players from Eaglehawk where he previously coached from 2003 to 2010, including to two flags, Filo said that was recruiting.
"Everyone pinches everyone's players... it is what it is," Filo said.
"I haven't coached there for three years. The reality is I'm the Mount Pleasant coach and when players of that calibre are on the move, you're negligent if you don't go after them."
Mount Pleasant is a traditional power of the HDFL, but hasn't played finals for five years.
BLUES ON THE RISE: Mount Pleasant veteran Trevor Douglas, recruits Matt Gretgrix and Kain Robins and key defender Chris Black. Picture: PETER WEAVING