Hi Deniska,
I understand your GPSviewer is set up for Holux GPSlim236 so it must be configured at 38400 baud. I does not seem to be configurable.
Is there a version for general serial GPS at 4800 baud?
I made up a cable for my old yellow extrex to use MapThis. At first I could not make it to work. I spent hours and hours trying to figure out why and then it daunted on me the change the config.txt to 4800 baud and voila!