As far as I know, there is no known method to obtain a full core dump file for iOS apps that you can later load on GDB.
Are you already asking your testers to send you the crash logs? Read this otherwise:
Understanding and Analyzing iPhone OS Application Crash Reports
Basically:
When the user synchronizes their device using iTunes, crash reports are
copied to a directory on the user's
computer. […] Depending on the
platform, the directory is:
Mac OS X:
~/Library/Logs/CrashReporter/MobileDevice/
Windows XP: C:\Documents and
Settings\\Application
Data\Apple
Computer\Logs\CrashReporter\MobileDevice\
Windows Vista or 7:
C:\Users\\AppData\Roaming\Apple
Computer\Logs\CrashReporter\MobileDevice\