MY,
It has been awhile since I made a MON08 interface for a GP32. As I recall there are 3 pins which must be in the correct state when the reset pin goes inactive. These pins specify the baud rate, baud rate source, and boot mode of the GP32. If the boot mode is MON08 then the GP32 will be available to connect serially using pin PTA0 to your MON08 host and receive commands.
I found a schematic of a GP32 production board I made and attached it to this response. As you can see, when the CPU08 programmer is attached, the PA7 and PC1 pins are pulled low and the PC0 and PC3 pins are pulled high.
In production none of the diodes were stuffed and a bed of nails fixture was used for program and test.
Good Luck,
Peter