What if Discipline target get bounce by Gust, can Discipline controler target an other unit ?

No — once Discipline is on the chain its target is locked in, so you can't move the +2 to a different unit; you still draw the card.

Targets are chosen when the spell is finalized, not at resolution, and can't be redeclared (Rule 355.15). If Gust resolves first and returns your unit to hand, the unit is no longer a legal target, so the "+2 might" instruction is ignored — but "Draw 1" is a separate, unconditional instruction and still resolves (Rule 359.3.e.6, FAQ #2489).

Note this only works one way: if the Discipline has already resolved and your opponent casts Gust afterward, Discipline's buff makes the unit 4+ might, which is above Gust's "3 might or less" restriction, so Gust would have no legal target and do nothing (FAQ #36).