On-site lightning impulse (LI) test waveform, restricted by the large load capacitance and impulse generator's inherent inductance, has a long wavefront time Tf compared with standard LI, called non-standard LI. The non-standard LI test has been carried out in the field. But its insulation defect detecting effectiveness is not clear. In this paper, a generating system of impulses with different waveform parameters, including double-exponential impulses with different wavefront times in the range of 0.08 ∼ 23.5 μs and an oscillating LI with wavefront time around 10 μs, was established. The insulation characteristics of SF6 gas gap with highly inhomogeneous electric field under impulses with different waveform parameters were investigated. Experimental results show that the insulation defects detecting effectiveness of oscillating LI with long Tf around 10 μs is lower than that of standard LI. Tf plays an important role in the insulation defects detection. With Tf increases, the 50% breakdown voltages for rod-plane gaps have an increased trend. The V-t curve presents a U-shape. Prospective voltage is introduced to evaluate the insulation defects detecting effectiveness. The prospective voltage-time characteristics confirm the conclusion that with the increase of impulse voltage Tf, the detection effectiveness for GIS insulation is reduced. The gap distance has influence on the shape of V-t curve. With distance increase, the 50% breakdown voltage of the gap is more sensitive to the wavefront time Tf and voltage rise rate dU/dt. Corona stabilization effect is not just in relationship with the rise rate of impulse, but also with electrode structure and gas pressure. The critical dU/dt for corona stabilization is calculated for different rod-plane gaps.