The continuous wavelet transform (CWT) has been utilized as an effective and powerful time-frequency analysis tool for identifying the rapidly-varying characteristics of some dispersive wave signals. Particularly, in the applications of continuous Gabor wavelet transform, its effectiveness is strongly influenced by the shape of the applied Gabor wavelet so the determination of an optimal shape tracing well the time-frequency evolution of a given signal. Since the characteristics of signals are rarely known in advance, the determination of the optimal shape is usually difficult. The main objective of this work is to propose a method to determine the signal-dependent optimal shape of the Gabor wavelet for the best time-frequency localization. To find the optimal Gabor wavelet shape, the notion of the Shannon entropy which measures the extent of signal energy concentration in the time-frequency plane, is employed. To verify the validity of the present approach, a set of elastic bending wave signals generated by an impact in a solid cylinder are analyzed.