Agreed. The trace command is in my opinion a necessity for troubleshooting post processor issues.
For those that are not familiar with this check out the below cut and paste from the my gpp file. Please note that there are 6 levels of tracing. 0 is none up to 5 which is the most complete. Putting the ; (comment) in front is equivalent not having the ; and making the level 0 or no tracing. Per the example below you can trace the entire output with levels 1-5 or specific calls with levels 1-5 to pinpoint a problem in a specific call.
All you do is go into your editor, open your gpp file, turn on tracing, generate G-code as usual, view the output with tracing, and then turn it off when you want normal G-Code.
; GPPL variables
numeric_def_f = '5.4'
integer_def_f = '5.0(p)'
gcode_f = '2/2.0(p)'
mcode_f = '2/2.0(p)'
xpos_f = '5.4'
ypos_f = '5.4'
zpos_f = '5.4'
feed_f = '4.4'
tool_diameter_f = '5.4/1'
blknum_f = '5.0(p)'
blknum_gen = FALSE
blknum_exist = FALSE
blknum = 10
blknum_delta = 10
blknum_max = 3200000
; trace "all":1
; trace "@change_tool":5
;trace "@drill4x_pnt":5
;trace "@tool_path_info":5
; trace "@rotary_info":5
;trace "@fourth_axis":5