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Anyway, it should work alright, if you have a GC-to-USB adapter. I haven't read anything bad about them, but they are kind of cheap, and I'm somewhat wary of going 3rd party for non-USB stuff. Personally, I'd just drop the cash for real GC controller + the official GC controller adapter for the Wii U, as both have been extensively tested with Dolphin.
Anyway, it should work alright, if you have a GC-to-USB adapter. I haven't read anything bad about them, but they are kind of cheap, and I'm somewhat wary of going 3rd party for non-USB stuff. Personally, I'd just drop the cash for real GC controller + the official GC controller adapter for the Wii U, as both have been extensively tested with Dolphin.