This chapter concerns with the investigation of k-almost Ricci-Yamabe solitons and gradient k-almost Ricci-Yamabe solitons in perfect fluid space-times and generalized Robertson-Walker space-times. First, we deduce the criterion for which the k-almost Ricci-Yamabe solitons in a perfect fluid space-time is steady, expanding or shrinking. Then we establish that if perfect fluid space-times admit a gradient k-almost Ricci-Yamabe soliton with Killing velocity vector, then either it represents phantom era, or the gradient k-almost Ricci-Yamabe soliton is expanding or shrinking under some condition. Moreover, we illustrate that a generalized Robertson-Walker space-time represents a perfect fluid space-time if it admits a k-almost Ricci-Yamabe soliton. Next, we establish that if a generalized Robertson-Walker space-time allows a k-almost Ricci-Yamabe soliton of gradient type with constant scalar curvature, then it also represents a perfect fluid space-time.