FLT CREW OF A320 ON APCH TO ORD FORGET TO EXTEND GEAR AND GET GPWS WARNING. EXTEND GEAR AND LAND.

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CAPT WAS PF AND FO WAS PNF. BRIEFED APCH TO RWY 4R AS ADVERTISED ON ATIS. DURING BRIEF; PF SAID TO ASK TWR IF WE COULD ROLL TO THE END; SINCE IT IS 8000 FT. ON ARR; APCH TOLD US TO EXPECT RWY 9R. TURNED A SHORTER THAN NORMAL FINAL AND WAS ADVISED WE WERE FOLLOWING A B767. PF ELECTED TO HAND FLY THE APCH AND SHORTLY AFTER; HEARD THE B767 ADVISING TWR HE WAS GOING AROUND. THINKING THE WX WAS LESS THAN ANTICIPATED ON ATIS; PF RECONNECTED AUTOPLT CLOSE TO DEANA (FAF). PF REQUESTED FROM PNF TO ASK TWR TO ROLL TO THE END THINKING IT WAS RWY 4R. TWR WAS CONFUSED AS TO WHY WE NEEDED TO ROLL TO THE END UNTIL WE REALIZED OUR MISTAKE THAT RWY 9R WAS LONGER AND WE DIDN'T NEED THE WHOLE RWY. THROUGH THE CONFUSION; WE DID NOT EXTEND THE LNDG GEAR. MADE THE REQUIRED 1000 FT CALL AND SOON AFTER THE 'TOO LOW TERRAIN' GPWS ANNOUNCED. I IMMEDIATELY PUT THE GEAR DOWN AND RAN THE FINAL DSCNT CHKLIST. THE PF SAID HE STILL SAW DEANA ON HIS NAV DISPLAY (POSSIBLE MAP SHIFT). A GAR WAS SUGGESTED BY THE PNF; BUT THE PF SAID HE HAD THE RWY IN SIGHT AND WE WERE IN A POS TO LAND. THE LNDG WAS UNEVENTFUL. CONTRIBUTING FACTORS WERE DISTRS AS TO WHY THE PRECEDING AIRPLANE IN FRONT OF US WENT AROUND; DISTR CAUSED BY THE CONFUSION OF ROLLING TO THE END INSTEAD OF REALIZING RWY 9R WAS 2000 FT LONGER. NOT ADVOCATING NEAR ENOUGH SINCE WE WERE NOT IN THE DEFINED STABILIZED APCH CRITERIA AND HAD A GPWS WARNING. POSSIBLE MAP SHIFT CAUSING A DETERIORATION IN SITUATIONAL AWARENESS -- WRITTEN UP IN THE LOGBOOK AND MAINT ADVISED. SUPPLEMENTAL INFO FROM ACN 617417: CAPT VIOLATED SOP BY NOT GOING AROUND AND TAKING TIME TO DETERMINE IF GPWS ALERT WAS VALID. I BELIEVE AN XA00 WAKE UP CALL HAS SOMETHING TO DO WITH BAD DECISION MAKING.