In this paper,we investigate the gap plasmon excitation by swift electron beams in nanowire pairs, in particular, with different cross-section shapes (including circular, wedge and irregular shape). For all these nanowire pairs, our results show that fast electron beams can efficiently excite low-order monopole-monopole gap plasmons, which possess long propagation distance and well spatial localization. Furthermore, in contrast to high-order plasmons, the excited gap plasmons do not depend highly on cross-section shape of nanowire pairs through comparing the merit parameters in these three kinds of waveguides. Besides, the excited gap mode will be more confined in nanowire pairs with wedge cross-section. Our study will be helpful for the experiments of gap plasmon generation using electron microscope.