The Generalised Frequency Division Multiplexing (GFDM)is a new multicarrier waveform that has been proposed in order to meet the requirements of the applications that are expected to work over fifth generation (5G)wireless communications systems. Due to its multicarrier nature, the performance of GFDM systems is highly sensitive to non-linear distortions. In addition, due to its wideband characteristics the memory effects of the non-linearities must also be considered. Thus, this paper presents closed-form analytical expressions that allows to evaluate the spectral efficiency (in terms of out-of-band emissions)of GFDM systems operating over a non-linear channel with memory. These analytical expressions are used to obtain numerical results for many different parameters system, allowing a quantitative evaluation of the spectral performance.