The transient grating technique has been developed over the years and several reviews have been written concerning the subject [1,2]. The present paper will focus on its advantages for studies of main native defect in semiconducting GaAs crystals, namely, of EL2, which plays a crucial role in semi-insulating properties and optical absorption below band-gap, photorefractivity, metastability, etc. We will consider the role of EL2 in optical nonlinearities and its contribution to carrier transport in subnanosecond time domain. Nonlinear character of TG technique allows to get deeper insight into spatial distribution of growth defects (EL2 and dislocations) in the wafers and perform their nondestructive monitoring. Different experimental techniques, as DFWM in nano- and picosecond time domain, light selfdiffraction, and set of differently grown GaAs crystals enabled us to show applicability of this technique for basic research and nondestructive determination of parameters, defect distribution, reveal fast transients of optical nonlinearities. The photorefractive nonlinearity at 300K is shown being dependent on dislocation density due to local strain fields around charged dislocations. Moreover, transient quenching of EL2 by short laser pulses and enhancement of low-temperature photorefractive nonlinearity is demonstrated.