FLC EXECUTES GAR IN AN A320 DUE TO WINDSHEAR ALERT.

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EXECUTED GAR DUE TO WINDSHEAR ALERT; FLAPS 3 DEGS; GEAR DOWN. NO OTHER ACFT RPTED SHEAR. AFTER COMPLETING GAR; PRESSED PERFORMANCE KEY TO ACTIVATE AND CONFIRM APCH PHASE. CAPT DIRECTED ME TO PERFORM REVERSE Z. LOOKED AT FLT PLAN PAGE TO CONFIRM APCH WAS IN; SAME WITH RADNAV PAGE. LOOKED AT PROGRESS PAGE BUT DON'T REMEMBER CHANGING ANYTHING. DID NOT GO TO PERFORMANCE PAGE AGAIN. ON FINAL; CAPT WITH AUTOPLT AND AUTOTHROTTLE ON FOR POSSIBLE WINDSHEAR. MANY TFC CALLS; BUSY RADIOS. TOLD MAINTAIN 170 KTS; FOLLOWING HVY; AND HVY DEP BEFORE LNDG. GEAR DOWN; FLAPS 2 DEGS. CAPT WENT TO MANAGED SPD. ACFT WENT FROM 170 KTS TO APPROX 190 KTS. HE STATED ACFT WASN'T SLOWING DOWN. ASKED IF HE WANTED SELECTED SPD; SAID NO. RAN FINAL APCH CHKLIST. CAPT ASKED ME TO SELECT LNDG CONFIGN. PULLED UP PERFORMANCE PAGE ON MCDU TO DO SO. NOTICED ACFT STILL IN GAR MODE. ACTIVATED AND CONFIRMED APCH PHASE. TOLD CAPT SPD SHOULD BE COMING BACK. AT SOME POINT HE HAD CLICKED OFF AUTOTHROTTLE/AUTOPLT. GOT 'TOO LOW; TERRAIN' DUE TO FLAPS 2 DEGS. CAPT ELECTED TO CONTINUE. WHEN SPD PERMITTED; FLAPS 3 DEGS SELECTED. SPD CAME BACK RAPIDLY. I THINK SPD WAS WITHIN 10 KIAS OF VAPP AT TOUCHDOWN; AND I THINK IT WAS IN ZONE. EXITED RWY. PROCEEDED TO GATE WITHOUT INCIDENT. MAY NOT HAVE COMPLETED APCH ACTIVATE ON DOWNWIND; NOT SURE. MAINT RPTED BOTH FMGC'S NEEDED RESET AT NEXT PWR-UP.