It is proved that Gabor's expansion of a signal into a discrete set of Gaussian elementary signals exists. An expansion into another discrete set of functions is defined, which functions are biorthonormal, to the Gaussian elementary signals. Hence, the expansion coefficients of the two expansions can be determined easily.
http://ieeexplore.ieee.org/iel5/5/31288/01455955.pdf?tp=&arnumber=1455955&isnumber=31288