GPWS ALERT TO AN LJ45 FLT CREW WHEN ALT XING RESTR NOT MADE AT FAF DURING LOC APCH RWY 6 AT TEB.

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WHILE FLYING LEAR 45 IN IMC AFTER VECTORS TO LOC FOR RWY 6 AT TEB; I DSNDED BELOW STEPDOWN ALT BTWN DANDY (MANDATORY 1500 FT) AND TORBY (MINIMUM 1300 FT). AFTER DANDY; I BEGAN MY DSCNT TO MDA OF 520 FT WITHOUT LEVELING AT 1300 FT UNTIL TORBY. DISTANCE BTWN DANDY AND TORBY IS 2.2 MI ON APCH PLATE. THIS WAS AFTER HDOF TO TEB TWR FROM NEW YORK APCH. GPWS GAVE OBSTACLE WARNING; BUT I CAN'T RECALL WHETHER THIS WAS BEFORE OR AFTER TORBY. I DO THINK IT WAS AFTER THE STEPDOWN FIX; AS I WAS AT MDA OF 520 FT WHEN THE WARNING OCCURRED. 2 OBSTACLES ARE NOTED ON EACH SIDE OF APCH COURSE AFTER TORBY (APPROX 370 FT ON L AND 420 FT ON R). I WAS ON LOC WITHIN 1/2 DOT THROUGHOUT THE APCH. THE CLOUD BASES WERE RAGGED AND WE MADE VISUAL CONTACT WITH THE RWY AT APPROX 2 MI AND LANDED UNEVENTFULLY. THE GS WAS NOTAMED OTS AND I WAS AWARE OF THAT IN MY PREFLT PLANNING. WE SET UP NAVAIDS; COURSES; FMS; AND BRIEFED THE APCH AT LEAST 25 MINS IN ADVANCE; WHEN WORKLOAD WAS LIGHT. OUR FMS WAS LIMITED; AND UNABLE TO DO APCHS; AND WAS SUBSEQUENTLY DEFERRED PER MEL PRIOR TO FLT. THIS LIMITED OUR ABILITY TO HAVE GS AND STEPDOWN INFO THAT WE NORMALLY USE AND TRAIN WITH. MY CO-PLT WAS HOLDING THE APCH PLATE AND DID NOT REINFORCE THE NEED TO LEVEL AT 1300 FT FOR THE STEPDOWN. SOMETIME BTWN THE BRIEF AND THE EXECUTION; I HAD FORGOTTEN ABOUT THE STEPDOWN. THIS; COMBINED WITH OUR LIMITED FMS ABILITY; CONTRIBUTED TO MY DSNDING EARLY TO MDA.