Bad gain is a term referring to a servant unable to refund 100% under the ideal support, for Ranmaru case being DCC. If a servant is unable to achieve good gains under ideal conditions then for those who are not lucky enough to provide her with necessary buffs, Ranmaru will definitely be underperforming.
However for well established players, Ranmaru’s bad gain doesn’t hamper her loopabilities that much. She can still 3T the regular 3x3 nodes with the correct MC.