Readied actions happen after their trigger
A Gust readied with the trigger "an enemy tries to use a melee attack on my party" can be used only after the attack is done. See the rules for Ready in DnD Basic Rules on DnDBeyond:
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
If your trigger is "an enemy comes into striking range of my party", you can fire your Gust before they attack. However, if they still have movement left, they can use that to close in to your party and land the attacks as normal.
Finally, if the enemy can make multiple attacks and you decide to trigger your Gust by "an enemy attacks my party", you can fire the spell after the first attack to move your enemy (possibly) outside reach. Beware that if the enemy has movement left, they can use it to close in to land the remaining attacks by the Moving between attacks rule:
If you take an action that includes more than one weapon attack, you can break up your movement even further by moving between those attacks.
If the enemy had to spend the last of its movement closing in to your character, they cannot close in anymore after Gust fires, potentially costing them the opportunity to attack at all.