You need sdf file (standard delay format) file which can be obtained by running static timing analysis on the design using Synopsys Primetime tool or doing synthesis of the design using Synopsys Design Compiler. Once you have sdf file. You can run it on the synthesized gate level netlist by putting the following in your test bench file
initial
begin
$sdf_annotate("filename.sdf",top_level_instance_name); //for example $sdf_annotate("./design/gate_level/vga_enh_top_ps.sdf",test.UUT)
end
This will do the back annotated timing simulation
Hope this helps