Monolithic integration of iontronic devices is a key challenge for future miniaturization and system integration. The G-Cap, a novel iontronic element, is a switchable supercapacitor with gating characteristics comparable to transistors in electronic circuits, but switching relies on ionic currents and ion electroadsorption. The first monolithic in-plane G-Cap integration through 3D-inkjet printing of nanoporous carbon precursors is reported. The printed G-Cap has a three-electrode architecture integrating a symmetric “working” supercapacitor (W-Cap) and a third “gate” electrode (G-electrode) that reversibly depletes/injects electrolyte ions into the system, effectively controlling the “working” capacitance. The symmetric W-Cap operates with a proton-conducting hydrogel electrolyte PVA/H₂SO₄ and shows a high capacitance (1.6 mF cm⁻²) that can be switched “on” and “off” by applying a DC bias potential (-1.0 V) at the G-electrode. This effectively suppresses AC electroadsorption in the nanoporous carbon electrodes of the W-Cap, resulting in a high capacitance drop from an “on” to an “off” state. The new monolithic structures achieve high rate performance, reversible on-off switching with an off-value reaching 0.5 %, which even surpasses recently reported values. Establishing technologies and device architectures for functional ionic electroadsorption devices is crucial for diverse fields ranging from microelectronics and iontronics to biointerfacing and neuromodulation.