Dear Rabbi; I just got a cast iron grill top for my gas barbeque. It could have been used for non kosher food. Can I kosher it and how?

Shalom, Thank you for your question. Because nonkosher food may have been roasted directly on the grill, the grate must be heated until it glows (libun gomur) to be properly kashered. This can be done either with a blowtorch (be careful to make sure that you know how to use one properly!) or by sandwiching the grates between charcoal briquettes and setting them on fire. If the gas flame can be turned high enough, and the grate moved close enough to the flame, in order to heat the grate red or white hot this is also sufficient. Blessings.