The 51-year-old singer - who finished filming for 'The Masked Singer' UK in October and was revealed as Harlequin over the weekend - was left in pain for "about a month" after contracting COVID-19.
She told The Sun newspaper: "I’m slightly asthmatic, so initially it was just a case of feeling like I was breathing through a pinhead, but it was only at times I had to use my inhaler.
“The worst symptoms for me were the headaches. It was excruciating and constant. I was taking painkillers but after a while I had to stop because it wasn’t doing anything. It wasn’t even touching the sides.
“To have a headache like that was awful. I couldn’t even raise my head from the pillows. I was glad to get through, it was about a month of feeling really unwell. But there are people who had it so bad, and those who didn’t make it.”
Gabrielle - who recently dropped new single 'Stop Right Now' - is set to release her new album 'Do It Again' on March 5.
The new collection will feature songs she performed on the reality show, as well as "five favourite covers" and two new tracks.
She added: "I am so excited. On this album I’ve got five covers from the show. I loved doing the show so much — when I wasn’t hot, and just singing the songs.
“Taking songs from 'The Masked Singer' and singing them and covering them was just perfect. I’ve also got five favourite covers that I love, and I’ve added two new songs.
"So I’m just really excited. I’m looking forward to people hearing it for the first time.”