With the new mistargeting rules: When I play Gust on a viable target at Abandoned Hall and the spell is reacted to, so that the unit goes to more than 3 Might. If I have a unit there at the same time, does my unit get the +1 from Abandoned Hall although Gust doesn’t have a viable target on resolving?

Yes — your unit still gets the +1. Abandoned Hall's trigger cares only that a spell was played, not that it had a legal target when it resolved. Gust mistargets (Rule 359.3.e.7 — the instruction doesn't execute), but per Rule 359.3.e.10 a spell with no effect "is still considered played," so the trigger condition is met.

Timing also works in your favor: the Abandoned Hall trigger is placed on the chain only after Gust finishes resolving (FAQ 11036/11415), so your unit is still sitting there and is a legal choice when the trigger resolves. You choose the unit as the trigger is finalized, so you can pick whichever unit you control at that battlefield.

One caveat — this only holds because Gust resolved. If it were countered instead, the spell is not considered played and Abandoned Hall would not trigger at all (FAQ 10646).