Gin has bragged he had a girlfriend once to Gorilla , As its in the past tense and by translation they are not together . But he was bragging to Gorilla on his experience with women . Some speculate its the female from original Yoruzuya .Most people confuse her with Stalker Ayame .But he has mentioned twice about his girl . First time was in the past and second time
When Gorilla was saying he has experience with girls . Gin reminds him brothel girls don't count . That time gorilla shoots Gin has zero experience with girls too . At that moment gin clearly states he had a lover in the past .... The Arc where Shinpachi correspond via letters to Kirara (sickly girl poses to be her sister) or some other arc ... But remember the incident .
Gin-chan had a girlfriend It seems
Heh heh Do you see that? But it's very good that you love him even if he was a womanizer