I've got one called a "mayflash adapter" that I've had for the better part of a decade. I think it was originally a 3rd party wii peripheral, but it has slots for 4 GC controllers and dual USB with a switch for "PC/WII". It still works perfectly on dolphin and even steam across the various distros I've used over the years.

I just use the wiiU Gamecube USB adapter and OG gamecube Controller Works great with dolphin

I use the 8bitdo ultimate controller. It is super programmable, the analogue triggers work with dolphin, and it has a nice dock. It has Bluetooth and a 2.4 GHz dongle. 8bitdo also makes a wireless gamecube adapter that connects to an original GC controller. Although I haven’t tested it and it doesn’t mention Linux compatibility

There are a few good answers here already, so instead I'll go ahead and ask: Why a GC controller in the first place?

This is just my opinion, but unless you're going for absolute authenticity over any kind of comfort, I'd just use any modern controller instead. Unlike the Wii Nunchuks, the layout maps to modern controllers pretty well, particularly to Xbox-style controllers. The GC doesn't have that many exclusives either that would make a purchase worthwhile, since many of them got ports/remasters/remakes over the years.

So my advice would be to start emulating using a regular controller first, and only grab a GC controller after a while, if you still feel the need to get one at that point.