The Witches' Gloomix is a plot element.
The Gloomix takes the form of a glowing necklace or wrist cuff with a single gem on the end. The Trix receive the Gloomix from Lord Darkar, and use it. So far, the only known witches to earn this are Icy, Darcy, and Stormy.
At the end of Season 2, when Darkar betrays the Trix and sends them to Oblivion, they merge together into the Mega Trix and three more Gloomix can be seen on them: a yellow zone and two green leg bracelets.
Gloomix is the witch version of Charmix, given to them by Lord Darkar an dit makes them more powerful. However, while the Fairies obtain their Charmix naturally as the second level of their magical progress, it has not been precised whether other witches can also have a Gloomix or if only Lord Darkar can give one to a witch.
Shorly after Darkar's defeat (possibly within a week, since the end of year party occurred in the last episode of Season 2, and the Winx were already packing for vacations in the first episode of Season 3), the Trix were captured in Realix by the authorities of Magix and sent into the Omega Dimension, and the Gloomix had completely disappeared, perhaps since Lord Darkar, who had created them, had been destroyed.