Can I gust Yellow LeBlanc when she attacks my unit?

No — once LeBlanc, Fragmented (yellow/Order) attacks, she's a legal target for nothing at 3 Might or less.

Her base Might is 3 and her printed [Assault] is Assault 1 (Rule 807.1.b.3), so the moment she gains Attacker designation she's at 4 Might — above Gust's "3 Might or less" threshold. Assault is a passive, not a trigger, so there's no chain window to Gust her before it applies (FAQ #2011, Rule 807.1.c).

The only exceptions are windows where she isn't an attacker yet: moving to an open battlefield (no combat), or a chain opening from another unit's move trigger before combat is staged (FAQ #2215).