What happens if you gust a temporary unit?

The Temporary unit is returned to its owner's hand normally. There's no special interaction that prevents or alters this.

Gust reads: "Return a unit at a battlefield with 3 might or less to its owner's hand." Temporary (Rule 816.1.b) is a triggered ability that reads: "At the start of this permanent's controller's Beginning Phase, before scoring, kill this." Since Temporary only triggers while the card is a permanent on the board, once Gust resolves and the card moves to hand (a non-board zone), there is no permanent left to be killed later. The Temporary trigger hasn't fired yet — and if it is already on the chain, it would mistarget since the unit is no longer on the battlefield (Rule 359.3.e.7).

In short: Gust works exactly as it does on any other eligible unit. The Temporary keyword does not protect against being bounced.