GameCube controllers are now supported by Switch as of the console’s latest 4.0 update.
According to Eurogamer…
“You need the GameCube controller’s USB adaptor, but with it the Switch now recognises GameCube pads and already supports them for play.
Controllers show up labelled as “USB”, although other USB controllers such as the Xbox 360 pad do not work, GameXplain reports. Nintendo has specifically enabled GameCube pads.”