http://www.vgchartz.com/yearly.php/?reg=World&console=&date=2002&maker=
http://www.vgchartz.com/yearly.php/?reg=World&console=&date=2003&maker=
Not sure how accurate this is but it says that Gamecube sold over 7m buy the end of its first full year and with the way things are going Wii U will likely be around 5.5m by the end of the year, give or take a few 100k. By the end of 2003 GC was at 13.4m so in order for Wii U to match that it will have to sell about 8m in 2014.
Now there are some things to take into consideration, GC was $199 at launch and $149 within 6 months wo at any gicen point it was $50-100 cheaper than its rivals vs Wii U being $299-349 in its first year or $50-100 more expensive than its rivals. Now with PS4/One out its $100-200 cheaper than its main rivals so maybe going into next year that will make it seem like a better option.
Also Gamecube had a better release schedule from 1st/3rd parties and didnt suffer nearly as big a drought as Wii U. In the launch window GC had Luigi's Mansion, Smash Bros Melee, Rogue Squadron, Wave Race, Pikmin, Super Monkey Ball as key exclusives and in the first 3 quarters of 2002 had Sonic Adventure 2, Resident Evil Remake, Eternal Darkness, Mario Sunshine, Star Fox Adventures, Animal Crossing. In the holiday quarter it had Mario Party 4, Metroid Prime, Resident Evil Zero, Super Monkey Ball 2 and Sonic Mega Collection also throughout the year it had a majority of multiplat releases.
Wii U had New Super Mario Bros U, Nintendo Land and ZombiU as key launch exclusives. Exclusives in the first 3 quarters of this year included Lego City Undercover, Game & Wario, Pikmin 3, New Super Luigi U, Wonderful 101 and only a fraction of multiplats. Things have picked up in the holiday quarter with Wind Waker HD, Sonic Lost World, Wii Party U, Mario & Sonic Olympics and Mario 3D World and better multiplat support.
So Gamecube was cheaper than the competition and had strong support from both 1st and 3rd parties. For the majority of this year Wii U has been the most expensive option and had terrible support from both 1st and 3rd parties but In the last few months that has changed and Wii U sales have been increasing, we wont know until early next year if thats just a holiday boost or if there really is increased demand in Wii U.
As for needing 8m next year to match GC sales thru 2003, I think its possible. It may recieve a true price cut next year (this years price cut was more like a bundle realignment) and has some big games lined up like Donkey Kong, Mario Kart and Smash Bros which all are typically 5m+ sellers, also some smaller games like Bayonetta, X, Yarn Yoshi, Sonic, SMTxFE if spaced out nicely can help give Wii U a consistant release schedule next year and Nintendo has some unannounced games coming next year. Its not certain but i think its possible Wii U starts making up some lost ground against GC next year.
With the way Wii U is currently tracking it looks like its on pace to sell about 15m lifetime but if it has a solid turnaround something like 30m isnt out of the question.