Hi

Gangster x doctor
https://www.mangago.zone/read-manga/afterglow/

Gangster x Gangster: https://www.mangago.zone/read-manga/the_mad_dog_found_out_i_m_his_own_kind_team_hazama/
Gangster x Cop: https://www.mangago.zone/read-manga/can_a_gangster_leave_the_gang/
Assassin x Gangster: https://www.mangago.zone/read-manga/a_kiss_for_each_face/

https://www.mangago.zone/read-manga/first_call_1_the_virgin_surgeon_is_about_to_be_married_to_a_young_yakuza/

Wondering what exactly falls into gangster - maybe you like one of those

https://www.mangago.zone/read-manga/codename_anastasia/

https://www.mangago.zone/read-manga/kimi_no_yoru_ni_fureru/

https://www.mangago.zone/read-manga/kurayami_no_ry_wa_tonbo_wo_idaku/

https://www.mangago.zone/read-manga/roses_and_champagne/

https://www.mangago.zone/read-manga/cover_up/

https://www.mangago.zone/read-manga/max_mojave_s_case/

https://www.mangago.zone/read-manga/park_hanhoo_s_manager/

https://www.mangago.zone/read-manga/tasuketa_yakuza_ni_nerawaretemasu/

https://www.mangago.zone/read-manga/legs_which_cannot_walk/

Gangster like psycho or killer also fine? I have a lot more of them in my lists.
https://www.mangago.zone/read-manga/when_the_killer_falls_in_love/

Did you want especially bl? Or also hetero fine?

https://www.mangago.zone/read-manga/just_twilight/

https://www.mangago.zone/read-manga/the_mafia_nanny/

https://www.mangago.zone/read-manga/hotaru_no_yomeiri/