All-dielectric, phase-gradient metasurfaces manipulate light via a judiciously designed planar distribution of high and low refractive indices. In the established design approaches, the high-index elements play a dominant role, while the electromagnetic field existing between these elements is routinely viewed as either an incidental by-product or detrimental crosstalk. Here we propose an alternative approach that concentrates on exploring the low-index materials for wavefront shaping. In our Si metasurface, the low-index air gap between adjacent Si fins is judiciously tuned, while the high-index Si fins only have a single size across the whole metasurface. These gap modes provide the full 2π phase coverage, as well as high and relatively uniform transmission, at the deep-subwavelength scale. These characteristics are ideal for mapping a steep phase gradient, consequently suitable for high-efficiency and large-angle wavefront bending. This light manipulation capability is exemplified with numerical simulation in PW-SW (freely propagating wave to surface wave) conversion, where the wavefront is deflected by an angle of 90°. In the gap-mode meta-converters, the average unit size can be only 1/60 of free-space wavelength, an order of magnitude smaller than that of conventional all-dielectric metasurfaces. Their conversion efficiency can reach 68%, the highest value reported for any all-dielectric gradient metasurface THz converter.