Assuming i have a valid spice netlist, Does anyone know how can i generate a gds?

If your netlist has the right sky130 devices, then you can get a minimal amount of automation by importing SPICE into magic through the "File -> Import SPICE" menu item. However, that does not do any placement and routing, it just drops all the devices into a layout with the same cell hierarchy, and adds pins. The placement and wiring then needs to be done manually.

This is the holy grail of analogue layout automation.