B757-200 CREW HAD A GPWS TERRAIN WARNING AT MMGL.

Narrative

WHILE LEVEL AT ASSIGNED ALT OF 9000 FT MSL ON A RADAR VECTOR FOR THE STRAIGHT IN ILS TO RWY 28 AT GUADALAJARA (GDL) MEXICO (MMGL); GDL APCH CALLED 'FLT X (GARBLED) 8000 FT (GARBLED).' MY FO REPLIED 'UNDERSTAND FLT X CLRED TO 8000 FT?' GDL APCH REPLIED 'AFFIRMATIVE AND (?) MAINTAIN 180 TO LOC INTERCEPT.' I INITIATED A DSCNT TO 8000 FT AND SLOWED TO 180 KIAS. WE BELIEVED THE DSCNT TO 8000 FT WAS APPROPRIATE BECAUSE THE GS INTERCEPT ALT IS 7200 FT. IN THE VICINITY OF THE GDL 105 DEGS/14 NM POINT WE GOT A GPWS 'TOO LOW TERRAIN PULL UP' WARNING. I CLICKED OFF THE AUTOPLT AND STARTED A CLB AT THE TIME WE WERE VMC WITH ALL TERRAIN AND THE ARPT IN SIGHT. AT NO TIME WAS THERE A CONFLICT WITH TERRAIN. PASSING APPROX 8300 FT THE GPWS WARNING STOPPED. GDL APCH THEN CALLED AND SAID 'FLT X MAINTAIN 8500 FT UNTIL ON THE GS; CLRED FOR ILS RWY 28.' APCH AND LNDG COMPLETED WITHOUT FURTHER INCIDENT. GDL APCH NEVER QUESTIONED OUR 8000 FT ALT SO I ASSUME IT WAS CORRECT. THE APCH PLATE SHOWS A 7119 FT HILL IN THE VICINITY OF THE GDL 105 DEGS/14 NM. I ASSUME THIS CAUSED THE GPWS WARNING -- IF THE DATABASE IS CORRECT. AT NO TIME DID WE HAVE A MAP SHIFT; AND PNF (FO) WAS IN RAW DATA TO VERIFY OUR POS.