In fact, i never hard of a [Soldering Welding Iron before. So how do they work on soldering GPIO pin to circuit boards
Hopefully you’ve got it sorted by now, but FWIW my experience is that you won’t need a fancy soldering iron: any decent one will do, but make sure you get GOOD solder and dip the pins (yes, dip the whole block of GPIOs) in flux for a perfect job.
Even the best soldering iron becomes useless if its tip is grubby (or worse, cleaned with sandpaper!), the solder is not right and flux is not used.
Remember that the solder has to flow in the gaps, does not have to “drip” into.
Hope this helps