i have not tried GC particularly, but the wii u itself on some friends's 4k tv.
i myself only own a 1080P which is enough for me, but i have to say that wii u games look sharper on a 4k tv.
other than sharper, i have not looked too deep to say 'it looks much better than a 1080p tv'
though i wish i did own a 4k tv instead !
it was only some 9months ago where my bro bought a TV for me and i was so mad that he did not get a 4k tv for me instead as he paid over 900$ for a LG 47' 1080 p with webos.
i mean even at that time there were 47' 4k tvs for probably less than 900$.
only the 55' 4k tvs cost about 1k here :/
oh well, i am satisfied with my 1080 tv, but if you have or are planning to get a 4k tv, then defintely do that.
4k tv will just be good future proof for you.