Disclosing new details about her role, the Royle Family legend also confirmed that her matriarchal alter ego will clash with shopkeeper Rita Sullivan in the coming weeks.
Speaking to the Radio Times, Johnston teased: "She's the sort of person who takes pride in shooting from the hip.
"She wastes no time in ruffling feathers when she arrives in Weatherfield, which she thinks is a dump.
"As well as being blunt to the point of rudeness, she's also a terrible flirt, which is terrific fun and a great combination to play."
The 68-year-old added: "I've just filmed some scenes with Barbara Knox because Rita is one of the first people Gloria goes into combat with, and she was wonderful.
"Because the characters are so familiar and I've watched the show as a fan, I have to make sure I call people by their real name."
Gloria Price, the mother of Rover's Return landlady Stella (Michelle Collins), upsets Weatherfield locals upon her arrival next Wednesday (September 5) as she turns the pub into a B&B.