Walter Gregg was a psychiatrist who specialized in sleeping disorders.
Doctor Gregg focused his study as a psychiatrist on sleeping disorders and eventually took on Jeremy Frost as one of his patients for a sleep study. Jeremy was a young man who lost the ability to dream after his abusive father hit him in the head with a baseball bat. As part of his study, Doctor Gregg used African Dream Root which not only restored Jeremy's ability to dream, but granted him the ability to control and manipulate the dreams of others. At some point, possibly realizing how addicted Jeremy was to the dream root, Doctor Gregg cut off Jeremy's supply of dream root.
Having gone insane from his inability to dream, Jeremy grew desperate to be able to use the African dream root without Doctor Gregg stopping him. To this end, Jeremy used his new abilities to enter Doctor Gregg's dreams and kill him in his sleep.
Doctor Gregg's murder drew the attention of hunter Bobby Singer whose investigation led him to question Jeremy. In response, Jeremy trapped Bobby and later Sam and Dean Winchester in nightmares in an attempt to kill them. Jeremy insisted to Sam that he only killed Doctor Gregg because he was taking away Jeremy's ability to dream again, something that Jeremy was desperate to keep.