Prison setting.
Female led/driven.
Strong sister bond the driving force behind characters (Claire in CB, Mo Eun in TPOC).
Female leads framed/falsely convicted for the crimes they're sent to prison for (Bell in CB, Ahn Yun Su in TPOC).
Both series thrive outside the bounds of a typical GL. It almost feels like story comes first and the GL aspect comes second which makes it even more satisfying to experience. Both are not just great for a GL, but a series in general.