Well, Mzansi Magic has explained why.

Gladys was arguably one of Rockville’s most loved characters who was played by award-winning actress Brenda Ngxoli, but since the return of the fourth season, there has been no sign of Gladys. Which is disappointing considering how she ended season 3 on a high note.
Speaking to TVSA, Mzansi Magic explained that her character has always been a cameo role, aiding all major storylines within the show. 
The channel further explained that in the story Gladys met someone and moved to Witbank to open a tavern.

Quite ironic considering that Gladys was an alcoholic. It's also rather unfortunate, Gladys made the show ten times more entertaining. 
 Hopefully, the producers of the show will flip the script and return Gladys to her beloved viewers.

Source: zalebs
Image Credit: Instagram