They have a very similar feel. Both are dramas with unpredictable stories, dark plot, mysterious antagonists and plot twists.

In both the ML (played by Lee Jong Suk) is falsely accused of a crime that he didn't do and he tries to prove his innocence by exposing the real villain.

The main characters in both dramas are corrupt lawyers that do dirty work for bad guys. But then, suddenly they have to go through some sad and shocking experiences, that make them realise what is really important and they decide to become a better person for the sake of their beloved ones.

The main difference is that W2L has fantasy elements. But apart from that there are many similarities between these touching dramas.

In both dramas the male leads are rich and successful, but socially awkward guys, who suffer from phobias/allergies, that are a result of childhood traumas.

The female leads are not the ones, who they pretend to be, but they're the only ones, who gain trust of the socially awkward MLs and who can heal them from their traumas.

The relationship between the leads develops from hate to love.

Many funny and heartwarming scenes in both.

Both dramas are about being young and having dreams, when all life seem to be still in front. However, in GBC it is also shown, that not all dreams can come true and your life can turn out completely different than you imagined.
GBC and EW both have similar kind of humour, but also include touching and sad moments.
Also, if you liked Lee Joon Ki in Eulachacha Waikiki, in GBC there is a very similar character played by the same actor, Lee Yi Kyung.
GBC is more about family, while EW is more about friendship, but in general they have a similar feel to them.

They might be not that similar in plot and story, but have a similar feel to them.
There are many crazy/funny moments in both on the one hand, and at times they get really dark and thrilling on the other hand.
In both dramas the main antagonist is a psychopath.
And in both there are some funny scenes with gangsters, who look tough at first, but turn out to be softies actually and respect the main characters (ML from PD, and FL from SWDBS), after they get beaten by them.

Both series address child abuse, domestic violence and childhood traumas, as well as therapy and healing. The Female Lead of CON and the Male Lead of IOTL have similar mental issues.

There are shocking plot twists in both series. However, CON is a pretty dark psychologic thriller, while IOTL has romance, many light-hearted and funny moments in it.

Both are dramas with healing stories, where the male protagonists are business heirs, who suffer from mental disorders/barriers . They have a high position in big companies and get involved in dark intrigues and secrets. But despite that, they manage to stay warm-hearted, and they care a lot for the people they love.
The female leads have similar personalities; both are optimistic and cheerful, but both experienced tragic events in the past. They help male leads to become stronger and to overcome their mental problems.