One of the things you should immediately do when encountering Gippal is filch all the gil you can. He carries roughly 15,000 gil (a stark comparison to Baralai, who only has 300). Any capable thief should be able to handle the job, and once done feel free to change the sphere to, say, a Dark Knight (or healer, if you have yet to load one).
Now, it's important that you use an alchemist versus a white mage; alchemists don't heal through magic, so when Gippal uses his hush grenades you don't have to worry about losing your source of magic. Be careful once his HP falls below 5000 -- he will attack more frequently, and with quite a bit more zest. The Dark Knight's Darkness ability is invaluable in this battle, though non-elemental item usage is also helpful.
Your main goal in this battle is to take him out as quickly as possible once he reaches 5000, and heal shortly before the battle is over. You won't get a better opportunity between now and when the next battle begins.