You can't use original cartridges in Gateway mode. Unless you meant Classic Mode? In classic mode even less encryption checks are patched out. I doubt you'd ever get banned only using retail carts in classic mode. Unless you are using Classic Mode for it's intended purpose...playing out of region games on your console. (Japan/Europe games on USA console, etc). But there's no confirmed bans from people who've only played out of region legit games. Only those using public headers or those who have used public headers in the past, then got banned later even after they stopped.
You're a dead man walking. If you used public headers on that 3DS at any point in the past, it's still possible you will suddenly get banned tomorrow. Simply not using them anymore isn't enough. They might still have your console logged in their system and simply haven't gotten the ban for you processed yet.
As for me, I've only ever used my private Alpha Sapphire header with my Alpha Sapphire dump and for a little while used that same private header on a Omega Ruby dump. I have since converted those to CIA soon after I had sold my Alpha Sapphire game to GameStop. (so I'm no longer in possession of the game that has this header).
So if I ever get banned, I would consider it the first confirmed case of a CIA user getting banned. I haven't used public headers EVER on this 3DS XL or even the older 3DS I used to have that got bricked. (and that one is even more "legit" then the 3DS XL. That one has only ever run CIA files and my private Alpha Sapphire dump! Haven't even had a copy of Omega Ruby on that one yet)
Cearp confirmed in another thread that spoofing console ID is possible. However he has not personally tested this as he doesn't have a banned console to test it on.
Either way though, if you are a Sky3DS user on latest firmware, you're ***ked if the console gets banned. You can't spoof console ID using a Sky3DS because it requires that you exploit the system to modify the firmware (usually done on emunand) and currently there is no entry point/exploit for 3DS on latest firmware. Must be on 9.2 or below and it will be that way for possibly another 2-3 years.