This contribution presents a photonically excited modulated metasurface array, which enables one dimensional beam steering with a medium-to-high gain in the sub-THz range. The designed array consists of four rectangular sub-arrays separated 1.16x lambda(0), with lambda(0) being the free-space wavelength at 100 GHz. This spacing permits a scanning range of +/-5 degrees without grating lobes. The sub-arrays radiate at broadside and are excited at their center by a simple substrate integrated waveguide (SIW) circuit. The excitation consists in a Uni-Traveling-Carrier photodiodes (UTC-PD) connected to a wideband DC biasing network. The structure at hand exhibits 26 dB of gain with a 3% relative gain bandwidth, enables power-combining and it is easily scalable for higher gains.