The implementation of the Type-C Port on the GPD WIN does not work properly with things like this:
What is the practical issue that you get if you try to connect the GPD WIN to your ASUS MB169C+ monitor via the type-C port with that adapter?
There are three problems, one is that the GPD Win does not support Displayport alternate mode on the USB-C port. This means that cheaper adapters like the above will not work as they rely on this mode and do not include chips to generate the video signal themselves.
The second problem is that the GPD Win can't output much current over USB-C before the voltage starts to droop causing all sorts of weird problems. This causes issues with bus-powered devices like some hubs.
The third problem is a lack of USB-PD support. When I connect my PD charger to the Win while it's on, it works but defaults to 5V 3A dumb mode, that's fine. When I connect it while the Win is off it boots up far enough to say it's charging, then something happens and the Win shuts off. It then boots again to say it's charging, shuts off again, it loops forever. It drains the battery more than it charges to do this so if left overnight you wake up to a dead device. This does not happen with chargers that are only capable of 5V 3A dumb mode, only with smarter PD chargers.