Phosphatidylglycerophosphatase involved in the biosynthesis of cardiolipin (CL), a unique dimeric phosphoglycerolipid predominantly present in mitochondrial membranes and which has important functions for cellular energy metabolism, mitochondrial dynamics and the initiation of apoptotic pathways. Required for the stability of respiratory chain supercomplexes and for growth at elevated temperature, in presence of ethidium bromide or in absence of prohibitins. 1,2-diacyl-sn-glycero-3-phospho-(1'-sn-glycero-3'-phosphate) + H2O = 1,2-diacyl-sn-glycero-3-phospho-(1'-sn-glycerol) + phosphate Phospholipid metabolism; phosphatidylglycerol biosynthesis; phosphatidylglycerol from CDP-diacylglycerol: step 2/2. Belongs to the GEP4 family.