What exactly does X equal to? About to play Gwyvorn Unleashed.

The card itself should say on the front, with "Pierce 2" or whatever. The reason it says "X" on the back is that it's used on other cards, too, where the value may be different, and in some cases, for regular monsters, the minion and master versions may have Pierce but different values.

Yes. Specifically, the front of the card should give a value, such as "Pierce 3", which lists the amount of pierce the ability has. The general explanation of the ability on the back of the card uses a general symbol, X to represent whatever number is on the front. The linked arachyura have different amounts of pierce for the master and minion. Using "X" lets FFG just describe the ability once for both monsters.