A Garden Farmer is an in almost every case hihgly racist person who farms the Destiny 2 raid "Garden of Salvation" hence the word "Garden Farmer".
The most use of the word Garden Farmer is to insult a highly racist person, whether or not they actually farm the Destiny 2 raid "Garden of Salvation" doesn't matter in most cases.
Garden Farmers also like to discriminate every group of people that can be discriminated against.