The self-aligned gate (SAG) transistor architecture is attractive for electronic circuit applications due to its enabling attributes, including low parasitic capacitances and higher frequency operation. However, SAG transistors often rely on complex manufacturing, which limits their practical utilization. Herein, we overcome this bottleneck and demonstrate organic SAG transistors in which the self-aligned source/drain (S/D) electrodes are separated by the gate (G) terminal with sub-20 nm gaps. The SAG architecture eliminates parasitic overlaps while minimizing access resistance for the injected carriers. Moreover, precise work function engineering of the self-aligned Au S/D contacts is demonstrated using phosphonic acid (PA) self-assembled monolayers (SAMs) functionalized directly onto Au. Analysis of the Au surface corroborated by Density Functional Theory calculations and scanning tunneling microscopy reveal the unexpected formation of PA SAMs directly onto Au for the first time. Combining different organic semiconductors with appropriate SAMs enables the development of hole and electron-transporting SAG transistors with enhanced performance. Integrating the n- and p-channel transistors yields complementary logic circuitry with high gain and noise margins, showcasing the effectiveness of this approach. The work highlights the enormous potential of combining the SAG transistor platform with work function modifying PA SAMs to develop printed electronics with improved functionality.