Will gach, cubes etc. soon become illegal?

Choosing to take a risk to get a good item is the definition of gambling

Which is exactly what the cubes are. You take a risk to get the preferred stats you want, in addition to a possibility of "Upgrading" the rarity (Rare->Epic->Unique->Legendary). If anything, it's TWO "Gambles" in one.

And what's the DFO method?^

You pay NX to choose the stats you want.