Kosinski, 37, shared the happy news on Instagram on Wednesday.
"Ready for the next chapter...I love you," he captioned photos of him and Gruman, 26, smiling in a snowy location.
Gruman wrapped her arm around Kosinski's shoulder with her square-cut ring on display.
The Challenge Mania podcast host proposed to the registered nurse at Coopers Rock State Forest in West Virginia where the couple likes to go hiking, they told Us Weekly.
Kosinski expressed how his relationship with his now-fiancée grew stronger amid the coronavirus pandemic while she works in Long Island, New York as a registered nurse.
"She's the real champ," the former Challenge champion told the outlet. "I've been home recording podcasts while she's been fighting a real-life horror movie-like pandemic."
He added, "I love her, I'm proud of her and I admire her."
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"It would suck to have a really nasty breakup, especially when you've created a lot of years of love with somebody," the Challenge: Double Agents star tells PEOPLE
The Challenge star also told the outlet Gruman has a great relationship with his son, Derrick Jr., 12, whom he shares with his ex-wife, Amy Manchin.
"Nicole and my son have a great relationship and laugh together like two goofballs, taking turns making weird sounds and faces," he said. "I'm excited for a fun-filled future together."
In true reality television fashion, the couple of three years were introduced by fellow Challenge star Jenna Compono.