Not sure about EMX/GCC but for kLIBC/GCC (basically all GCC versions above 3.3.5), yes it uses HLL debug info and mostly works in the IBM debuggers. I understand that there are slight problems at times with how the stack is handled and dmik had to work around GCC's object format of AOUT not having big enough fields for some of the HLL stuff creating warnings like,
20:00.87 emxomf warning: Input file 'K:\var\temp/ccTjZZujasXXXXXX' has more HLL
debug types than we can index in PUBDEF and EXTDEF records.
but it generally works in the debugger. Unluckily I don't really know much about using the debugger.