The charger that came with my GS500 is rated at supplying eight volts; is it safe to charge my shears that are only 3.6 volts on the charger?

Yes, the chargers have an internal diode and resistor designed to step down the voltage, when necessary ,and be used for multiple units. For example, the GS500 is a 3.6 volt grass shear, while the charger puts out eight volts. The reason for the difference is because the same charger is also used to charge the DS600, a six volt detail shrubber. The internal diode and resistor step down the voltage to be used with both units.