Along with a few other features the Switch 4.0.0 firmware update has added the compatibility with the official GameCube Adapter for the Wii U. It is worth noting that the Mayflash adapter for the GCN was working back in June, but now we've got official support. The Mayflash only supported one controller, with the official adapter you can hook only the black USB cables to the Switch and use two adapters for a total of 8 GCN controllers on the Switch. No news on if this was on purpose or a just pure luck with the USB Audio patch. But its got everyone speculating on what it could mean. Everything from Smash to GCN Virtual Console.
These little devices were really hard to find when they launched, so finding one now is going to be even more expensive. I'd hold off before you spend money though. But if you want to throw caution to the wind here is a link for one on Amazon