Shut down gas for heat boiler but did not shut down electricity
If you just shut off the gas without shutting off the electrical, it shouldn't damage the system.  Basically the thermostat(s) will continue to call for heat, the boiler will try to fire up and fail, then the boiler should give up and enter lockout. The boiler has safeguards built in to handle a lack of gas, which should prevent damage to the system.