We report what we believe to be the first demonstration of stable gain-switched 845 nm pulse generation (Er(3+): (4)S(3/2)->(4)I(13/2) transition) by a weak modulated 1550 nm seed laser, from an Er(3+)-doped fluoride fiber pumped by a cw 974 nm laser diode. When the pump power of the 974 nm laser diode is set to a certain value, the injection of a weak (<1 mW) modulated 1550 nm seed laser into the 845 nm lasing cavity can depopulate the lower state (4)I(13/2) of 845 nm lasing, switch the gain to regulate the conventionally chaotic spiking, and generate stable synchronous 845 nm pulse trains with the repetition rate up to 100 kHz.