The net elevation gain will burn about 3kWh (1.5kWh per 300 meters), so assuming you have a brand new car with a 20C or greater temperature battery, then you have 21kWh useable minus 3kWh for your journey.
120km / 18kWh = 6.66km per kWh or 150 wattHours per km.
With no reserve, perfect battery, no heater, no wind, dry roadway, blah, blah, blah, you could drive 100km/h.
You'll note that I didn't factor the extra power consumed over the pass because you will get a bunch of that power back, but of course, there is some loss. So, let's say some speed below 100km/h.
Now, if your car is not new, or the battery not warm, then we would have to readjust for the actual capacity of the battery to start.
See range chart to see how I calculated this.