Gentleman Caller is a hidden achievement/trophy in Dishonored.
How to Acquire[]
All side quests for Granny Rags must be completed to earn this achievement. This means completing "Gentlemen Callers" during the mission High Overseer Campbell, the first side quest for Slackjaw in House of Pleasure ("Slackjaw's Missing Man, Crowley") to activate the last confrontation with Granny Rags during The Flooded District, and in the latter completing "Help Granny Cook Slackjaw."
Each of these quests have to be completed, which also means taking Granny's rewards to validate them.