The terms goomer, a.k.a. gummer, gommerman, gommerwoman, are synonyms for the practice of Hoodoo/Rootwork/Conjure and for professional practitioners.
The term goomer is also used as a verb, as in "she goomered him", to describe the practice of using magic on a person.
Although I am not quite sure of the origin of the term goomer, I do suspect that the word is related to the term "goofer", as in goofer dust. The term goofer is also used as a verb to describe using magic on a person.