If i get you right, you mean you want to extract the parasitics from the gds? If this is what you want to do, you need to back annotate the gds(known as ANNOTATED GDS, this is different from normal gds. find out more on this). Then call any extractor tool to extract the parasitics from this annotated gds.
U can use STAR RC as parasitic extractor...get the GDS of the cell that u want to extract and give the GDS as input to STAR RC..it will generate the gds netlist and then run parasitic extractiion...u can also give circuit netlist to get accurate results..extractor will generate a spf file where all the RC details will be present for each and every signal