The former Blue Peter presenter was due to battle it out with five other celebrities on Thursday night for a place in the final stage of the competition.
However, Gethin was “gutted” that he had to immediately pull out after suspecting he might have coronavirus when the final was filmed back in March.
Judges John Torode and Gregg Wallace were seen telling Gethin’s fellow contestants Riyadh Khalaf, Judi Love, Sam Quek, Phil Daniels, Amar Latif and Sir Matthew Pinsent that he was too unwell to compete for a place in the final three.
As the episode aired, Gethin explained the situation to his followers, tweeting: “Thank you for all your lovely messages re @MasterChefUK. I had symptoms of Covid-19 the morning of the next stage.
“Absolutely GUTTED... but it was the easiest decision.
“Would never put anyone at risk. Enjoy the next hour – these guys can cook!!!”
Thursday’s final saw YouTuber Riyadh Khalaf and Olympians Sam Quek and Sir Matthew Pinsent make it to the final three, where they were challenged to prepare a three-course meal in two hours.
Riyadh was later crowned the champion after impressing with a starter of monkfish scampi in beer batter with wild garlic mayonnaise and a main course of beef fillet with port shallots, celeriac cream and a Parmesan tuile.
He finished with a dessert of white chocolate fondant with rhubarb poached in elderflower liqueur.
Recalling his win, Riyadh told the PA news agency: “I felt like I might faint, vomit, cry or scream – or all at the same time.
“I was so sure it wasn’t me. I was so sure that I was trying to prepare myself for not winning by in my head saying another contestant’s name over and over and over – so that way I was ready to hear their name and not mine.”
He also spoke of the impact he hoped his on-screen presence would have on young LGBTQ people.
He said: “I think back to when I was a kid watching TV and I had virtually no positive role models who I could look up to.
“I really truly believe that who I am, my identity and my feelings, were a curse and something that needed to be fixed and weeded out, a sickness even at one point.
“I think if I had had just one person that was respected and admired and talented and just a nice person in an elevated position, I could have looked at that guy, girl or person and gone: ‘Wow I can do that too.’”
Riyadh follows in the footsteps of last year’s winner Greg Rutherford, as well as stars including Angellica Bell, Lisa Faulkner and Kimberly Wyatt.