If you put Haste before attack and heal, then your character will immediately cast Haste once it wears off. Then when Haste is done, your character goes back to attack, then heal whichever gambit you've prioritized.

One tip, don't have your characters have the same gambit set up. If possible, have your gambit set up use up items first for status effects (ex. blind, silence, poison, sap, etc) before casting cures since casting takes up more time and MP.

I never have my attack enemy gambit as #1. It's usually #5 or even up to #8.