If You Like Gap Dong:
You might want to give an awesome (and less well known drama) White Christmas a try.
Starring: Kim Woo Bin, Sung Joon, Lee Soo Hyuk, Baek Sung Hyun
White Christmas is what I think Gap Dong was going for in concept before all the crazy writing twists and bad female characters interfered. What happens when a normal person is put into the situation where they can do unspeakable things. Or as White Christmas's psycho puts it, "are killers born, or are they created?"
This is my go to drama when I want something well written and super suspenseful. This drama takes us to Susin High School, nicknamed "Prison High," it is an elite school attended by the top 1% of students in the country. Stuck in the middle of the mountains with no outside influences the only thing that the students do is study and wait for Christmas break to leave for a week to visit their families. This Christmas 7 students and a teacher remain and are confronted with a killer among them who is interested in finding out if monsters are created, or if they are born. With one of the best up and coming cast of young actors, it is an amazing drama that feels reminiscent of a good Agatha Christie murder mystery.
White Christmas Trailer
If You Like Hotel King:
You might enjoy "Scandal: A Shocking and Wrongful Incident"
Starring: Kim Jae Won, Ki Tae Young, Jo Jae Hyeon, Park Jung Chul
Devastated and feeling wronged for the death of his son, Ha Myeong Geun kidnaps Eun Joong and when he comes to his senses and tries to return him, he finds another child substituted in as the kidnapped heir. Eun Joong is raised by his "adopted father" with a steadfast idea of right and wrong (ironic isn't it), and grew up to be a policeman. Shocked to find out the truth that the father he loved so much was actually his kidnapper he decides on what his idea of family is and comes to terms with the horrible actions of both his kidnapper father and real father.
If you like Witch's Romance:
You Might Enjoy I Need Romance 3
Starring: Sung Joon, Kim So Yeon
I know that INR3 is not really an old drama, but when I was thinking of which drama felt the most similar, this is the one that comes to mind. If you want some suggestions for other older woman/younger man themed romances feel free to check out this old blog post of mine with other noona drama suggestions
Shin Joo Yeon is a 33-year-old fashion marketing director at a home shopping channel company. She's developed a tough prickly outer shell in order to succeed in the workplace, and has almost given up on the idea of true love after countless failures in the relationship department. Joo Wan is a 26-year-old songwriter and carefree spirit who returns to Korea after seventeen years living abroad. Up until the age of 9, he was practically raised by his mom's friend and her daughter, Shin Joo Yeon. She only remembers him as a kid she was forced to play with him when her mom was busy, while all his early memories from bath time, to play time, to learning how to tie his shoe, include her. Upon meeting again, he aims to heal Joo Yeon's jaded sense of romance (synopsis from MDL).
I Need Romance 3 Trailer (for the shorter trailer go to this link https://www.youtube.com/watch?v=NIA4FadGYOc)