A compact triple-layer series fed slot unit array using Groove Gap Waveguide (GGW) technology for 5G millimeter-wave applications is presented in this paper. The design employed a two-step coupling mechanism for the radiating slots. The aperture coupling design when combined with the groove gapWaveguide (GGW) technology allows achieving three different goals: First, a reduction of the array size in the transversal direction, so more unit arrays can be added without grating lobes. Secondly, low insertion losses with high radiation efficiency. And third, a reduction of the manufacturing complexity with good stability against fabrication tolerances of up to 0.1 mm, making the design feasible to be massively deployed. The antenna was fabricated and measured showing good agreement with the numerical simulation results, the antenna provides a peak realized gain of 12.15dB at the central frequency of operation and a fractional bandwidth (FBW) of 9%.