GPS modul. How to get speed and put it in variable?

I think something like this will do what you want:

Serial3.setup (115200, {rx: P0, tx: P1});
var lastData;
var gps = require ("GPS"). connect (Serial3, function (data) {
  console.log (data);
  if (lastData) {
   // do stuff here - eg. the difference between the two readings
    var dlat = data.lat - lastData.lat;
    var dlon = data.lon - lastData.lon;   
  }
  lastData = data;
});

Although as you say, you might be able to extract the data that you want from other NMEA data. For instance VTG - Velocity made good sounds promising