The Keeping Up With The Kardashians star's weight gain has made headlines in recent weeks, and was even a trending Twitter topic after pictures of him jetting to Europe for sister Kim's wedding emerged last month.
At the time he tweeted: "So I found out I was trending for being fat. Thank you all. It really made my day... I'm a grown man and will work on myself when I choose."
But now, it seems the reality star is ready to get back in shape, and he's enlisted Gunnar's help to do so.
In a chat with People magazine, the personal trainer said Rob, 27, is focused on a new weekly weight-loss plan, but it won't involve hitting certain numbers on the scale.
"I'm not big on measurements," he said. "Until a person looks in the mirror and is happy with themselves, you have not reached your fitness goals."
Despite starting a new weight loss regime being intimidating, Gunnar is fully confident that Rob can do it.
"Rob works out in beast mode. When I see him, we laugh, we have fun and gets after it hard at the gym. I know he can do this."
The new workout regime comes after Rob skipped Kim's wedding. Although he flew out to Paris for the pre-wedding celebrations, instead of attending the nuptials in Florence, Italy, he flew back to the US.
Sources have suggested it was his unhappiness at the weight gain that prompted his decision.
Whatever happened, it seems clear that Rob is in a better place now. Chatting with fans on Twitter this week, he retweeted a positive and uplifting comment from a follower, adding his thanks.
"Warching your shows & just want to say you're such an awesome, cute & generous brother!! Keep it up!!" said the message.
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