When you import GDS; make sure the layer mapping is correct.
If you are sure that your mapping file is correct, then check the signal label.
You do not have Power and/or Ground. (your layout does not have INV, that means no substrate connect to Power/Ground. Try to use metal text (or pin) to put label on your power bus. (such as VDD, VSS...). Becareful not to miss label the Power and Ground. If you have Power and Ground short together. You will have bigger problem.