If you want to limit the voltage you need a voltage divider. So for a nominal 5V gate from a 12V supply you’d have a couple of load resistors R1 and R2 across the positive busbar and the ground, with R1 amounting to 5/12ths of R1+R2. Thus assuming high impedance on the gate input (which is normal for analogue modular synthesis) the potential difference (ie voltage) across R1 will be 5V. Have the button wired so that until it’s pressed it breaks the connection between R2 and the +12V busbar, so the gate input will be pulled to ground through R1.
Caution: I’m a software guy, not an expert in electronics.