I don't think gateway has any support. They do provide software tools for backing up and restoring your NAND. And they do encourage you to back up your NAND when setting up your gateway. I don't think they provide any hardware service of restoring your NAND though.
Nintendo will help with unbricking, provided you bricked while doing an official system update (with an unmodded system
). I'm not sure whether they'll do it for free if you aren't covered by warranty. Sending a hard-modded console anywhere is a bit silly. Since if you have a hard-mod, you should be able to restore your NAND by yourself and probably had the sense to backup your NAND. But yea, I'd guess if you sent it to Nintendo they'd laugh at you.
Sky3DS has the possibility of you getting banned, if you don't use private headers for your roms. Arguably the safest choice would be to have a 9.2 console, get cubic Ninja, and use PastaCFW, but avoid installing any system CIAs. Or alternatively using Gateway or some other card/system that allows you to use Emunand and just play with emunand (which is completely safe).
Emunand requires no actual console system mods (though you can install an MSET exploit to make it easier to boot. This is completely safe). And once you can boot into emunand, you are 100% safe. Which is why gateway is a popular choice: leave system firmware on 4.5, then use gateway emunand as your "actual" system. Any messing up of emunand can be fixed by simply setting it up again.
Either way, as I said, bricking sounds a lot scarier than it actually is. Provided you don't install any weird CIAs, or mess with System CIAs, you are pretty much 100% safe. And again, bricking can happen EVEN WITH OFFICIAL NINTENDO UPDATES. It's not some magic weird thing that randomly happens with hacks. it's a perfectly normal predictable event that happens when you mess up a system install.
hell, you can even 'brick' your desktop computer. Install a bad bios and see how much fun you have getting it back into a working state.
Edit: For the record, Sky3DS can't "brick" your system because it doesn't actually install anything. It just makes a 1-to-1 copy of a cartridge rom. If you don't use private headers, this means it's the exact same 'cartridge' that every other pirate that downloaded that game is using. Which means you'll get banned. AFAIK, this is the same for gateway headers as well. CIAs are safe in this regard.