The G-Cap is an iontronic device that combines features of an electrical double-layer capacitor and a transistor, exhibiting switchable capacitance behavior. This functionality is achieved by introducing a third electrode with larger surface area compared to the electrodes of the working capacitor (W-Cap), allowing reversible ion depletion or injection based on the applied bias voltage. The in-plane G-Cap is manufactured using piezoelectric 3D-inkjet printing with a sucrose-based carbon precursor. Substituting liquid by solid state electrolytes expands the practical applicability of G-Caps significantly. The ion-selective conduction mechanism provides means to control and analyze the switching mechanism. This study presents Nafion™ and SPEEK (sulfonated poly ether ketone) as suitable electrolytes, both of which are commonly used in fuel cells and batteries. These cation exchange membranes utilize protons as charge carriers and act as solid-state electrolytes, eliminating leakage risks. With these electrolytes, the device achieved a switching behavior with capacitance reduction to 5.8 and 3.8 % in the off-state using bias voltages of -0.5 and -1 V, respectively. This advancement represents a significant step towards the realization of ion-based logic circuits, potentially bridging semiconductor microelectronics with ionic computing, bio-interfacing, and neuromodulation applications.