Let p, q be distinct primes satisfying gcd(p−1, q−1) = d and let D i , i = 0, 1, · · · , d−1, be Whiteman's generalized cyclotomic classes with Z * pq = ∪ d−1 i=0 D i . In this paper, we give the values of Gauss periods based on the generalized cyclotomic setsAs an application, we determine a lower bound on the 2-adic complexity of modified Jacobi sequence. Our result shows that the 2-adic complexity of modified Jacobi sequence is at least pq − p − q − 1 with period N = pq. This indicates that the 2-adic complexity of modified Jacobi sequence is large enough to resist the attack of the rational approximation algorithm (RAA) for feedback with carry shift registers (FCSRs).