qlandkarte accesses USB GPS devices via the ugen device driver. A scan
is performed by successively opening ugen device nodes (starting with
ugen0) until either your GPS device is found or until the ugen devices
are exhausted.
By default OpenBSD creates only two ugen device nodes, so if for
example, your device attaches as ugen4, you will have to create ugen2
through ugen4 manually:
cd /dev && sudo sh MAKEDEV ugen{2,3,4}