To Mycroft, he was Gregory. Always Gregory. At work, at home, in the office, in bed, in the morning at breakfast and in the middle of the night. He would always be Gregory. Someday he'd transition from "a colleague" to "my friend" to "my partner" to "Mr. Greg Holmes-Lestrade," but to Mycroft he was perpetually Gregory.
To Greg, he was usually Mycroft. At dinners, and dates, and at home, he was Mycroft. At the office, if he was helping on a case or in front of the other detectives, he was strictly Mr. Holmes. In the dead of night, when he got up for water and woke Greg up, he was Myc. Sitting next to him on the couch, watching telly and drinking whiskey, he was Darling (always a sarcastic nickname; Greg couldn't take himself seriously if he started calling anyone darling). He was Mikey once when he found some of Mycroft's baby pictures and toys from growing up, but that was just once. In bed, he was Mhh-mmm-m...m...Mycroft! In his head, however, he was mine.