!! WARNING !!
I am not responsible if you use this information to damage your device! Continue at your own risk!
After many hours of probing and writing to the EC Registers, I have finally found a way to control the fan inside the GPD P2 Max! You can even completely disable the fan 
You will need a program called NoteBook FanControl (nbfc) found here: https://github.com/hirschmann/nbfc
Then you will need the GPD P2 MAX config XML file I have created and attached to this thread. GPD P2 MAX.xml (2.7 KB)
Install NoteBook FanControl and then navigate to the directory where you installed it (default location is C:\Program Files (x86)\NoteBook FanControl). Inside you will find a folder called “Configs”, just paste the attached XML file into this folder.
Now just run the NoteBook FanControl program, click the three dots in the top right corner to select a config file, click the dropdown box, and scroll down to the GPD P2 MAX config!
A few notes about how this all works… GPD did something weird with the registers that control the fan. In most system you can find the register and write any value between 0 to 100 into it (sometimes 0 to 255), which corresponds to 0% to 100% speed. I could find no such register in this system. The only register I found that gave reliable control on the fan speed, seems to only accept a value between 0 and 7. So we end up with 8 total fan speeds (off, and 7 run speeds). Not ideal, but better than no control at all!
EDIT: Ok I just realized someone has already done all this in another thread lol. Oh well, the more the merry I guess
EDIT2: The config file attached to this thread is only compatible with bios version 0.21, it does not work with the newer bios version that have fan profile settings in the bios, as the registers use different values.