You must finish the original Duel before beginning the next one generated from Onslaught.
The only time a new Duel may interrupt the current Duel is with Slow to Die Actions.
So, in your scenario:
1: Lady Justice triggers Onslaught.
2: Gremlin triggers "Squeel!".
3: Damage is Dealt
4: Squeel! triggers and the Gremlin is pushed 4" directly away from Lady Justice.
5: Wounds are dealt.
4: Duel Ends
5: The new Duel would then occur from Onslaught, but if the Gremlin is no longer within Lady Justice's melee range, the Strike will not occur.