I encountered some problems. No bricking but thought I'd share just in case anyone encounters the same thing it could be helpful.
I followed all the instructions.
1. Updated an older 2012 3DS XL (older, but still new in box, just opened) to 4.5.0-10U.
2. I put all the files on a MicroSD for the blue card and put it into 3DS XL. On first attempt the loading screen was stuck so I tried again and the next try it loaded up fine and installed the exploit successfully. Hit "A" and powered down.
3. Put launcher.dat onto SD card in 3DS XL with Nintendo files after it was already formatted by the 3DS XL.
4. Booted into Gateway menu using "DS Profile + L" in settings. Menu came up fine.
5. Did the NAND backup. Process completed successfully.
6. (still in Gateway menu) I wanted to boot into GW mode so I could power down and get the NAND backup file off the SD card (like the guy in the video on Gateway's site does). So I put the Gateway cart into the 3DS XL (it demanded the cart be inserted when I attempted to boot into GW mode without it. weird, since that guy in the video booted Gateway mode without the cart inserted and THEN put it in), hit "A" to boot Gateway mode, but it told me I had to update the Gateway cart so I let it update the Gateway cart and it completed successfully.
7. I tried to boot Gateway mode after updating Gateway cart successfully (because it told me I had to) I got a system error from 3DS XL.
8. After powering down I then took out the SD card and copied the NAND backup file to PC and put SD card back without the NAND file on it. I tried to load Gateway menu, but could NOT load Gateway menu instead getting the previous system error. I checked and the firmware was still 4.5.0-10U.
At this point I thought "Do I need to reinstall the exploit or what?" But I didn't try that, instead I went back and copied all the 3DS XL SD card's contents to another,
different SD card and used it because the SD card I was using previously kept acting like it was locked when I'm certain it wasn't (it wouldn't delete the NAND backup after I had copied it to PC, there was no delete option) so I figured the SD card might be screwed up. After trying with a different SD card in the 3DS XL I could successfully bring up Gateway menu. I tried to boot into Gateway again, screen turned black for a while then it was in 3DS menu but Gateway cart was not showing up (apparently it doesn't until you hit select and load a rom). I didn't realize this until after another two attempts one of which had me hang on a black screen indefinitely and the second try finally had me booted into Gateway mode and I got it working.
I figure it hangs on a black screen indefinitely if the cart isn't making good enough contact or something. On my DSi XL my DSTwo has to be inserted a few times to be recognized occasionally if I've been taking it out for any reason. I'm just glad Gateway didn't decide to up and brick the 3DS XL with that issue happening (pretty sure it was the SD). I ran disk checks on every flash memory card I used after copying files to them yet still ran into that weird issue.