Debugging requires different tools based on what part of AmForth you want to debug. To debug CODEWORDs, or in situations where AmForth is generally not responsive (e.g. boot time and initialization issues) you will need an external debugger like GDB. External debugger is less convenient for debugging COLON words. This is because ITC is not directly executed, the CPU never jumps into the PFA area of a word, so you cannot just put a breakpoint there. For these situations AmForth also provides an internal debugger, which is the most convenient way to debug COLON words.
There are other AmForth or general Forth debugging techniques that can be useful in some circumstances. See AmForth Programming and Debugging for a list of such techniques.