We have not yet added support for this in the device functions (DeviceConfigure/DeviceRead/DeviceWrite), but this will be supported.
But it is possible to do this yourself as well, and it isn't very difficult.
Remember to launch the Wolfram Language as root (i.e.: sudo wolfram), because the GPIO modifications require root level access:
gpioExport[n_Integer] := Put[n, "/sys/class/gpio/export"]
gpioDirection[n_Integer, dir_] := Put[dir, "/sys/class/gpio/gpio" <> ToString[n] <> "/direction"]
gpioValue[n_Integer, val_] := Put[val, "/sys/class/gpio/gpio" <> ToString[n] <> "/value"]
And then, for example, this will turn on an LED if it is properly hooked up to GPIO13:
gpioExport[13]
gpioDirection[13,out]
gpioValue[13,1]