You must provide a path for the computer signals.
The LPT1 port provide 12 out, 5 input and the other 7 wires are signal return, which is at 0v (to the computer.
When you are connecting to the Geckos - the Geckos also require a +5volt supply to the input side, to power the opto electronics - and note I said on the input side - .
You therefore need a supply of 0-5volt.[color=Black] I would not recommend you using a voltage regulator off the 37,5 volts [/color] - because this is on the output side. You would be best to use a seperate mini supply which are readily and cheaply available for games consoles, phone chargers etc. Regulate that with your 5 volt regulator. Your input side is then completly seperate to your output.
The +5volt from your mini supply goes to the Gecko. The step and dir wires from your LPT1 socket go to the Gecko. The 0v lead from your mini supply goes to the signal return on the LPT1 socket (any one of the seven). This is necessary to give the Gecko a reference for the signal from the computer.
Your step and dir pins need to be active low since the are "pulling down" the 5 volt supply to the Gecko through the opto isolation diode.
It is not necessary to make any connection between the input side of you Gecko to the output (37v) side.