If Diana Lunari is gusted back to hand, does her ability still proc if there is no enemy on the BF, just my Diana?

Yes, Diana, Lunari's ability will still trigger and you can successfully resolve the effect even if she is "gusted" (recalled to your hand) in response.

Ruling Breakdown

1. Triggering: According to RiftJudge FAQ #9373, moving a unit to an open battlefield (where no enemy units are present) initiates a "non-combat showdown." This triggers Diana's "When a showdown begins here" ability.
2. Chain & Resolution: When a triggered ability is placed on the chain, it becomes an independent object. Per RiftJudge FAQ #9879, the ability does not require the source (Diana) to be present on the board when it resolves.
3. Cost Payment: The cost [1] is part of the ability's instructions ("do X to do Y"). As established in the same FAQ, costs within instructions for triggered abilities are paid on resolution. Because the ability is independent of the source, you can pay the cost and resolve the effect even if Diana has been returned to your hand.

Sequence of Events

1. Trigger: A showdown begins at the battlefield where Diana is present. Diana's ability triggers and is placed on the chain.
2. Response: The opponent plays a spell like Gust to return Diana to your hand.
3. Resolution: Gust resolves, and Diana returns to your hand.
4. Ability Resolution: The triggered ability remains on the chain. When it resolves:
   • You are given the opportunity to pay [1].
   • If you pay, you perform the [Predict] action and draw a spell if the revealed card is one, exactly as written on the card.

This interpretation relies on the mechanics of triggered abilities (as detailed in the retrieved FAQs) that resolve independently of the source unit's presence on the battlefield.