We present a modular THz-driven electron gun with both tunable interaction length and output orifice allowing optimization of the sub-mm interaction volume. First extraction of multi-keV electrons is demonstrated and the parameter space as well as resulting performance of the THzdriven gun by varying the timing of the two single-cycle THz pulses and the UV photo-excitation pulse are explored. Such compact photo-gun prototypes are not only promising as injectors for compact THz-based linear accelerators (LINACs) but also as source for ultrafast electron diffraction experiments.