Is Victron GX connected to NMEA 2000? You need a special cable for that (you could also make your own, see this article)
If you verified the connection, make sure your GPS source is turned on (it might be the chartplotter).
When everything is working you should be able to go to Signal K Data Browser and search for navigation.position and it should show your current position.
If you haven't already, do check this tutorial, it explains though: