Conversion routines only happen automatically where selection screens and writes are involved.
The _input function is called when screen fields are passed to the program, and the _output function is called when a variable is written (either to a report or to another field). In all other cases the conversion routines must be called explicitly.
So, when you populate your GT_Table you must explicitly call the _input version of the conversion exit with the output going to the relevant ID field.
If however, you are already using a parameter to enter your ID value, post the domain definition here and I'll look at it.
Regards
Rich