I assume that the gds file that you have is the IO ring, standard cell and macro gds file.
Step by step for you to start place and route:
1. Convert all the gds into LEF file. I am not aware that Cadence Encounter can help you to do this conversion. But you can always try Cadence ICFB tool. Remember only metal 1 layer and above needed in the LEF file.
2. Then you need to have netlist describe your design.
3. You need to complete your floorplan as well. Basic requirement for your floorplan before place and route will be the core area definition include the placement row definition, power planning.
4. So at this this point, you will have LEF file, design netlist, floorplan ready. From here you can start doing place and route using Cadence Encounter tool.