How can I say the Italian term "dislivello" in English?

Solution 1:

The difference between the lowest and highest altitude along the trail can be called "Elevation difference"

A frequently used term is "elevation gain" however this is an addition of all the ascents within the trail without taking into consideration descents.

If for instance the trail starts at 0m altitude, proceeds uphill to 200m, then downhill 50m and uphill again by 100m, the elevation difference would be 200-50+100=250m, while the elevation gain would be 200+100=300m

Solution 2:

I've seen simply rise and fall, used as nouns. This website of the Adirondack Mountain Club, for instance, has:

After about a mile of traverse and a rise of around 500 feet, the trail turns distinctly more up hill ...

Just a couple of sentences later it also uses ascent:

Near the top the trail becomes much gentler for the last sever hundred yards and the last couple of hundred feet of ascent.

You can also use the verb forms: the trail rises/ascends and falls/descends.

If you're going to use the word as a tag, in a table, I'd use rise/fall rather than ascent/descent, since ascent and descent usually mean the act of going up or down, what the hiker does. You could use just rise with a positive or negative number:

Rise: –120m