Can I place a cast iron skillet on a gas grill and heat it to 600 degrees without destroying the cast iron?
yes- heat away! Cast iron can be heated to wickedly hot temperatures..Those woks used at chinese restaurants are cast iron...they can take the heat & hold the heat for a really long time...just be sure not to submerge it in ice water when you're done.
As long as it doesn't begin to glow red you are probably okay. But how are you going to get it that hot in the first place? That's a lot of BTUs!