Ok, so that should work with GT2, assuming that U are using it in a PSX. I used to hybrid GT2 with a cdx 3.3 with no problem. U will need to use the enable code for that version of cdx, which should already be installed in the GT2 code section. Then as Mr C. said, U will need to determine the version of the game U have so U know which basecodes to use. One way to do this is to try the money code for each version till U find one that works.
Version 1.0
99,999,999 credits
801D0FC8 E0FF
801D0FCA 05F5
Version 1.1
99,999,999 credits
801D1338 E0FF
801D133A 05F5
Version 1.2
99,999,999 credits
801D1568 E0FF
801D156A 05F5