Gail Fleming is Sharona Fleming's younger sister, an actress. Sharona, Benjy and Monk stayed at her house after a small earthquake (Sharona chose to stay with her rather than with Adrian Monk because, to quote her, "I will go crazy slower at Aunt Gail's"). She and Sharona are constantly butting heads because Gail believes that Sharona copies her (moving to San Francisco, buying the same purse, etc.) (Mr. Monk and the Earthquake)
Gail joined the cast of a stage play that became a big hit and was picked up for a Broadway run. For this reason, she was framed for murder by her understudy, Jenna Ryan, and Jenna's father. She was denied parole by the Judge before Monk could give details about how she was innocent because the Judge had hard feelings for Monk from a prior trial. She was cleared with Monk and Sharona's help. (Mr. Monk Goes to the Theater)
Benjy often goes to stay with her when Sharona is called away on business or for personal reasons.