We study theoretically the viscoelastic relaxation of cross-linked copolymers in the framework of generalized Gaussian structures (GGS), which are extensions of the Rouse model to arbitrary geometries. The model holds for general homogeneous copolymers under free-draining conditions; it is especially suitable for treating alternating copolymers. We calculate the storage G'(omega) and the loss G"(omega) moduli both for un-cross-linked and for cross-linked copolymer chains, and observe a variety of features: alternating copolymers differ from homopolymers in the high-frequency domain, where G"(omega) may display two maxima. Cross-linking alternating A-B copolymer chains into regular networks (lattices) leads to the appearance of a network-dominated, low-frequency relaxation domain. In the case of very large differences in the mobility of the A monomers, of the B monomers, and of the cross-links, G'(omega) is very structured, displaying three relaxation domains, separated by two plateaus. G"(omega) shows three peaks. We expect that these features can be readily detected through appropriate mechanical relaxation experiments.