When relays are closed the current that supplies the armature solenoid is cut off. The solenoid forms an inductor and when that current stops the magnetic field created by the prevoiusly flowing current collapses and induces and equal and opposite current in the coil. With nowhere to go this current can create a spike of voltage as high as 1000 volts. In order to prevent this from damaging other components it's useful to place a diode in between the coil leads that allows the current to flow in reverse without the spike in voltage.
When selecting the diode we need to know what current that would be flowing through the coil when on, and then voltage applied to the coil. The Diode's forward current should be equal to the relay's coil current, and the Diode's reverse voltage should be at least equal to the relay's coil voltage.
In the case of the GL5E I'm using the coil current at 5 volts is 79.4 mA. With some buffers we should be safe with a diode of the following specs:
V reverse = 100V I forward = 100mA
The ON 1N4148 fits the bill with a reverse voltage of 100V and a forward current of 300mA.