A 3-D printed planar permittivity-gradient superstrate (PGS) is used to improve the directive radiation characteristics of a waveguide-fed compact resonant cavity antenna (CRCA). Far-field directivity of classical uniform superstrate-based RCAs is limited due to nonuniform aperture phase distribution caused by even transmission through the superstrate. Transverse PGS has been used here to remarkably improve aperture phase distribution and hence directive radiation performance of RCAs. Furthermore, the PGS was rapidly prototyped in one hour and 43 min using a low-cost acrylo-butadiene styrene (ABS) filament without using traditional multistep milling and machining. Single step fabrication was performed and effective dielectric constant of the ABS was varied through controlled infill percentage in different regions of the PGS. Measurements of a prototype indicate unrivaled results, from a smaller footprint, which includes peak directivity of 16.048 dB, 3 dB directivity bandwidth of 49.65% and sidelobe levels lower than -10.4 dB throughout the operating frequency band. The 3-D printed PGS thus outperforms all previously reported superstrates, for RCAs, by demonstrating similar radiation performance with an equivalent material cost of only 0.41 USD.