To plow a gagger is slang for snorting a large line of Cocaine or Crystal Meth. Plowing comes from the size of the line being too large to snort at a normal speed so the user accidentally 'plows' the line with the straw, moving the drug rather than snorting it up the tube. Gagger comes from the action of gagging that is often associated with snorting a line of this magnitude.
Dude, I just scored an eight ball! Lets go plow a gagger.