when I come in with two units vs Kha'zix but the opponent uses gust on one of my units does Kha'zix get the ability?

To determine if Kha'Zix, Mutating Horror receives his bonus in your scenario, we must look at the specific moment his triggered ability attempts to enter the chain.

Short Answer

No, Kha'Zix, Mutating Horror will not receive the +2 Might and 2 XP.

Reasoning

According to Rule 383.3.e and the structure of triggered abilities, the condition "if an enemy unit is alone here" must be true at the moment the trigger condition ("When I attack or defend") is fulfilled.

1. Trigger Condition Check: You move two units into a battlefield where Kha'Zix, Mutating Horror is present. Kha'Zix gains the "Defender" designation.
2. Condition Assessment: At the moment Kha'Zix gains the designation, the game checks his ability's condition: "if an enemy unit is alone here."
3. Result: Because you have two units at that location, your units are not "alone" per Rule 741.1 ("A unit is alone when there are no other friendly units at the same location"). Since the condition is false at the moment he gains the designation, the ability fails to trigger and is never placed on the chain.

Why Gust Does Not Help

In your scenario, you stated the opponent uses Gust on one of your units. For Gust to be relevant to this ability, it would have to be played before the combat designation is assigned or as a reaction to the triggering event.

• If you have two units present when Kha'Zix gains the combat designation, the trigger condition is evaluated immediately. Because the condition is not met, the ability does not trigger.
• Once the ability fails to trigger, the opportunity to place it on the chain is missed for that instance of gaining a combat designation.
• If your opponent plays Gust on one of your units after Kha'Zix has already gained the designation and the game has already evaluated (and failed) his trigger condition, the state of the board changes too late to retroactively trigger the ability.

As established in FAQ #9849 and #9937, the condition is checked at the moment the unit enters and gains the designation. If you have two units present at that critical moment, the "alone" requirement is not satisfied, and the ability does not trigger.