Gammer Brevis is a witch who teaches school over the mountain from Lancre. She takes snuff and does her own shoe repairs, which makes her "all right" in the opinion of Nanny Ogg. She has a nasty habit of being reasonable when provoked which in turn provokes seething fury in Granny Weatherwax. She appears as a coven member in Witches Abroad, discussing who should take over Desiderata Hollow's cottage with Granny Weatherwax, Nanny Ogg and Old Mother Dismass. She was the last person to see Mabel Peavey before her mountain-climbing accident.
A very similar witch is mentioned in The New Discworld Companion as Gammer Beavis. A witch of this name also appears in the short story The Sea and Little Fishes; these may be the same character since Beavis is described by Nanny Ogg as 'a bit too educated' which would fit with her being a teacher like Brevis.
Gammer is an old English word for a country woman.
The Sea and Little Fishes (a short story)