In reply to Kirankumar V Hiremath:
Your error is caused by extraneous and improper use of parenthesis.
@(posedge clk) ('1,current_time=$realtime)
|=> (clk_1G_period <= $realtime-current_time-0.001ns ||
clk_1G_period >= $realtime-current_time+0.001ns, $display("clk is 1G") );
That removes the error, but you probably did not want the $display in the property, you want it in the assertion only if it passes.
assert property (chk) $info("clk is 1G");