The GTA01 uses the u-boot bootloader.
More information on u-boot can be found at [1] and [2].
However, the vanilla u-boot doesn't support many of the features that GTA01 needs, such as
User:HaraldWelte is working on those issues, and in fact most of them have already been implemented.
The current bootloader source code can be found at [3]. You will need the original tar.bz2 archive as well as the u-boot-qt2410.patch.
Untar the sources, apply the patch. run "make qt2410_config", run "make". You will get a resulting "u-boot.bin" image, which you can directly flash (either using existing bootloader or sjf2410-linux) into NAND.
If you want to do bootloader development on the QT2410, it's easier to work with a bootloader image that can be downloaded via USB into RAM instead of flashing.
To do so, you need to edit the u-boot/include/configs/qt2410.h file, and change the "if 0" in Line 32 into a "if 1", then recompile with "make".
The resulting "u-boot.bin" is _NOT SUITABLE_ for NAND flash, but only for direct execution from within ram, e.g. by using the s3c2410_boot_usb program.