Faith (2012)

Both dramas feature time travel to a different era.
In Tunnel, granted it is just from 1986 to present.(still a big culture shock)
In Faith it is to the Goryeo era from the present.
Both bring a little star power with their Brave and Manly leading men.
Faith's cast is lead by Lee Min Ho playing a General of the army and bodyguard to the King.
Tunnel's cast is lead by Choi Jin Hyuk playing a tough Detective who is not afraid of tackling a dangerous criminal. He is also a respected sunbae for his less experienced partners.
Both have a lead who is faithful while longing for their loved one after being separated by time.
Both are directed by Sin Yong-hwi
Both are really good dramas worth watching.

Where they are different:
Tunnel has a serial killer crime story and slightly dated outfits.
Faith has political intrigue, medicine, and of course the long hair and historical dress.

Both are dramas with a modern day FL who is magically transported to historical times where she meets and falls in love with the ML.

Time travel, characters move between past and present. Both are Korean shows. ML feels hopelessly resigned to his fate until he meets the other (ML or FL).

Both are historical dramas with fantasy worldbuilding and elements. Both have a similar feel despite very different plots.

Both have people who time travel and fall in love with someone from a different time.

Historical setting, both are Korean dramas. ML and FL have a strong bond and good chemistry. ML becomes traumatized by the constant war. Surprising hopeful/happy ending after ML is wounded/near death and thought to be dead/possibly dead by FL.

if you like korean history dramas, and people with special powers, lee min ho, captivating story line, and just plain old good shows. its very similar to this show, except the story line is different. so basically if you like gu family book, you will like faith

Time travel, characters move between past and present. Faith has a more serious tone but there are similarities.

Time travel, characters try to change fate. Bittersweet but happy feeling ending. MYG is short length, while Faith is full length, though.

Both have female lead time travel
Both havd a struggling romance with happy end
Male lead falling in love first

The story content is different but both of the girl in the story went through time travel back to the past time.