The 68k calculators are indeed delightful machines. You've found TIGCC - you should also be aware of GCC4TI, a fork of TIGCC due to the original maintainer not

The 68k calculators are indeed delightful machines. You've found TIGCC - you should also be aware of GCC4TI, a fork of TIGCC due to the original maintainer not accepting contributions, and GTC-IDE which is an independent (non-gcc) C compiler that runs on the calculators themselves (although the frequent memory leaks render the development loop tedious - no memory protection means leaked memory is gone for good until you "reboot"). There is a mature independent OS called PedroM, and someone even made some progress porting 4.4BSD (PunixOS).