Good advice from joecaption, but if you happen to be located in a rural area, on propane, then there is no "gas company." If that's the case, here are a few things to think about:
Installing a gas line under an existing slab isn't impossible, but it can be challenging for a typical homeowner. You'd have to tap into the line where it enters the house, and then run a new line along an exterior wall and then down below grade at the kitchen. Directional boring would get you from the outside of the house to the kitchen target location you're shooting for, and can be done by a qualified plumbing outfit--many have rigs equipped to do so efficiently, but it will require digging a work-pit to enable access for the boring equipment. I've done several "amateur" borings under concrete slabs successfully, but the longest was only 5' or so. Used a half-inch electric drill, with a home-made spade bit, and lots of bit extensions. The gas lines I installed were black steel pipe, with threaded connections. And don't forget to do a leak-down test, to make sure all fittings are absolutely tight. You don't want an accumulation of gas under the house, waiting for the right event to go "poof"!
A far simpler approach might be to go through the attic with the gas line, and then down into the kitchen at your new island location. You could locate the line in some false kitchen columns, located at the corners of the island and abutting a short pony wall to define the end of the island. Or use just 2 columns in the middle of the island, one on each side of the range, to support the custom range hood the wife has been dreaming about forever.