Both of these shows explore the darker side of high school and present controversial themes. Gloomy Salad Days has more of a supernatural element by using death girl as the antagonist. Whereas LIFE shows the darker side of humans
The two of them are about Death who embodies a young girl. Also the both of them talk about the transition from the living world to the world after death.
Both shows are supernatural themed about gods of death. In Gloomy Salad Days, death girl goes around collecting souls of lost young people who no longer have the will to live. She also slowly falls in love with a boy who can see her. In Secret Angel, an angel falls in love with a human that she has to protect from a god of death. And death also starts to experience human emotions.
Similar theme about people that have made a pact/are approaching their own demise. The story about the Death Girl.