If you can downgrade, you don't need a Gateway, but if you're planning on playing a lot of games and trying out all the new titles as they get released, you may want to consider getting a Gateway because it's much more convenient to load ROMs than installing CIAs. To install CIA on the New3DS you need to shut down your system, unscrew the battery cover, take out your SD card and copy the CIA files to it, then put the card back in your system and screw the cover back on, then wait for them to install, which can take a long time for bigger games that are a few GB each. With a flashcart, however, you just take the cart out without shutting down, put a ROM on the SD card and pop it back in, then press select to load them instantly without having to install anything.
EDIT: You can also install CIA over wifi without shutting down and taking the SD card out, but it's super slow.