If your outdoor gas barbecue has a grill and burners made out of cast iron, you should season them carefully prior their initial use, and thereafter always keep them coated with oil to keep them in good condition and prevent rust. If you initially season and afterward care properly for your cast iron cookware, it should outlive even you!

[edit] Steps

Preheat your oven to 275ºF.
Wash and dry the grill and burners.
Cover them completely with a light to moderate coating of vegetable oil.
Place in the oven for thirty minutes.
Remove and let cool, then reinstall on your grill
Repeat

[edit] Tips

Spray to completely coat the cast iron with cooking spray after you have finished cooking and let the grill cool down.

[edit] Warnings

Clean the grill immediately after use. Some suggest running at a high temperature to burn off any remnants, while others suggest burning off the remnants at a high temperature will also burn off the seasoning and invite rust.
Do not ever wash cast iron with soap or use a scrat pad - that would take off the protective coating that you worked so hard to get.