"Unfriendly Number" challenge
Your divisors generation is lacking. As explained e.g. here, you should generate them from a number's prime factorization. It is much faster that way.

You may try using rem instead of mod.

Also try making (nubOrd $ map (gcd k) u) a separate expression and name it to ensure sharing, giving your function a concrete type signature too.

update so I tried my own advice and it only made the code run slower at the interviewstreet servers. The only reason I'm not removing this, is that the advice itself (and link) about divisor generation is valid in general. But here, it did not help improve the run time of this algorithm. The OP states that same algorithm in Java got through...