Not much is known about Grady's life.
Grady is a drug dealer who deals phenobarbital (and, assumedly, other drugs). He is under the impression that Des is using it because he is addicted to it.
In Bloody Mary, he intentionally waters down Des' supply, resulting in Troy Dixon's death being very painful. This was to test whether or not Des was using or selling it; when Des confirms he is using it, Grady does not believe him.