Gnuk supports the STM32 part of STM8S Discovery Kit now.
STM8S Discovery Kit consists of three parts: free soldering area, STM8S area, and USB area. At USB area, STM32 is used as a kind of USB controller to access STM8S from PC.
If you don't mind to break it, you can just split USB area from the Kit and use the board for Gnuk. The chip is STM32F103C8T6, which only has 64KB flash memory, but it is enough for Gnuk.
It is the cheapest board for Gnuk. In Akihabara, you can buy it just 750JPY.-
Provided you have some JTAG dongle, you can enjoy.
Pin layout of CN5 of the board is like this:
TnRST 8 7 GND TDI 6 5 TMS TCLK 4 3 TDO 1 +3.3V