  We present a numerical study of dispersion manipulation and formation of
matter-wave gap solitons in a Bose-Einstein condensate trapped in an optical
superlattice. We demonstrate a method for controlled generation of matter-wave
gap solitons in a stationary lattice by using an interference pattern of two
condensate wavepackets, which mimics the structure of the gap soliton near the
edge of a spectral band. The efficiency of this method is compared with that of
gap soliton generation in a moving lattice recently demonstrated experimentally
by Eiermann et al. [Phys. Rev. Lett. 92, 230401 (2004)]. We show that, by
changing the relative depths of the superlattice wells, one can fine-tune the
effective dispersion of the matter waves at the edges of the mini-gaps of the
superlattice Bloch-wave spectrum and therefore effectively control both the
peak density and the spatial width of the emerging gap solitons.
