You say you want a portable but then mention NG for a fuel source and 13 kW for size, both typically mean a stationary generator. The wiring, fuel and safety considerations are substantially different when connecting up a portable vs a permanently installed stationary genset.
If it’s a portable generator, the neutral will be bonded to ground internally and you’ll need to address that so it’s safe both when plugged into the house and when it’s running independently. Local AHJ here lets us leave the ground disconnected at the inlet socket, but not all AHJs will feel the same. You’ll need to have a plan and get their agreement. Whatever you do, don’t bypass the internal bond inside the genset unless you’re willing to remove all the sockets, weld the frame to your foundation or otherwise render it impossible to haul out to a field and run independently of your home wiring system.
If it’s a permanently installed generator, it will have provisions internal to either make or break the N-G bond, because it’s designed for the application.
If you’re really planning on running off natural gas (or propane), make sure you size the plumbing right. Generators take a considerably larger supply than most houses are plumbed for, you might need to upsize the regulator and service to keep from starving the engine when you need it most. Backyard ROT is 3/4” pipe minimum, if it’s a short run.
Interlock kits in the existing panel is one solution to adding a generator safely, but I’ve found they’re often more trouble than they’re worth - the panel needs to be new enough to support backfeed clips in the right places, you need to find interlocks that fit your specific breakers and panel, yadda yadda. A 60A or 100A double pole double throw safety switch is infinitely safer and usually cheaper/faster, easier to explain to the wife in case the power goes out when you’re out of town, and looks more professional to boot. Just my .02
