I'm gonna say never, barring a miracle (like Splatoon becoming the next Pokemon or Wii Sports).

The GameCube is actually going to increase its lead over the Wii U this year, because holiday 2003 was the GameCube's biggest holiday season. It had a huge spike thanks to a price cut to $99.99 and the November launch of Mario Kart: Double Dash.

2003 was the only holiday season where the GCN beat the XBox. 

So Wii U will continue to lose ground for the rest of the year.

I don't think Zelda U will cause that big of a bump either. It's not a big IP in Japan anymore, and isn't huge in Europe either. And Hyrule Warriors + Smash + Wind Waker HD will probably already bring a lot of the Zelda fans over before hand anyway.

In 2016 because the 2015 baseline would be bigger than what it is right now.

Plus......SMASH BROS.

I hope around 2017 but sometimes I think it won't ever overtake the Gamecube.

Never.

Someone get the chalkboard!

Its possible but not really likely. One thing to consider tho is that Wii U nearly 2 years after launch still has an official price of $299, while Gamecube has been $149 for over a year at this point and less than 2 months away from hitting $99. Maybe just maybe when Wii U gets a significant price reduction, it can start to make up some lost ground against Gamecube. That and if Amiibo turns out to be a massive hit among children.

So basically if Wii U hits $199 by this time next year and Amiibo becomes huge then I think Wii U has a chance to pass Gamecube but if that doesnt noticeably change its sales then I would say 16-18 million sounds reasonable.

Sometime in 2016. If not then or before, never.

not at the current price. maybe in a year or 2 from now like after zelda comes out.

2017. It will cross 30 million but not much more

The real sales GC is:

2,25m 2001
7,08m 2002
13,4m 2003
17,58m 2004
20,32m 2005
21,33m 2006
21,52m 2007

I think that Wii U will pass GameCube by september 2015.