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If running the command with rosrun as in your command line example works fine, I do not see why your roslaunch fails (provided both are executed by the same user in the same environment, otherwise there might be some problem with the device file permissions, or even the gps might associated to a different /dev/ttyXXX).

However, you may try setting the parameters using <param> tags inside the <node> tags (which IMHO is a better style anyway):

<launch> 
  <node pkg="nmea_gps_driver" type="nmea_gps_driver.py" name="nmea_gps_launch">
    <param name="port" type="str" value="/dev/ttyUSB0" />
    <param name="baud" type="int" value="9600" />
  </node>
</launch>

Maybe that makes a difference.