I never tried this myself, and as you indicated it might be tricky soldering those tiny pins, but this is what I would try if I was trying this:
Get one of these (or similar):
https://www.amazon.de/GP2X_or_whatever/dp/B0FPPR9QFK
Find one of these Samsung E800 (or compatible) cables with the 24pin connector:
https://www.amazon.de/-/en/LOLCOPTER/dp/B008I2D5YK
Some electrical wire, and some perfboard, or something else to solder a cable to. And you'll need the tools, like soldering iron and a multimeter.
The tricky part, a standard samsung USB cable only connects GND for you. We don't need/want the USB part so we ignore those.
You only need to connect pins 6 (RXD) and 7 (TXD), for that you would have to break open the connector itself. And I don't know what you'll find it there, it might be difficult to solder onto these pins.
As these are tiny cables I would use the perfboard (or similar) to solder the tiny cables to and solder a cable from the USB-UART adapter to that board. If a cable breaks off, you want it to be the bigger one not the one going to the connector.
You can use the multimeter to check which USB-pin / cable is connected to which pin.
Note: Use only the 3.3V pins on the USB UART plug.
PC-adapter: TXT <-->
PIN7 GP2X
PC-adapter: RXD <-->
PIN6 GP2X
PC-adapter: GND <--> GND (PIN1 or 2, or maybe even PIN22) GP2X (USB cable should already be connecting this)
Pin layout EXT port, if you look from front to the pins:
.........................................................................
|
01 02 03 04 05
06 07 08 09 10 11 12 |
| 13 14 15 16 17 18 19 20 21
22 23 24 |
.............................. \ .. / ....................................
Check your pin-out, after soldering, with a multimeter so double check you connected to the correct pins. Also check if you use 3.3V as higher might damage the GP2X.
You can use PuTTY with these settings to connect:
Baudrate: 115200
Data bits: 8
Parity: None
Stop bits: 1
Flow control: None