I need some help with a project where I am trying to operate a unipolar stepper motor with a raspberry pi's using the 4 GPIO pins and using a L293D IC as an amp to get it to the operating 12V of the stepper motor. The problem is that to use the method I have which is choose the + or - pairs of each coil on the stepper and switch between them whilst using the 5th wire as a common ground, I need the GPIO pins that are not set to HIGH to not become a ground connection. Any help would be appreciated or another method that I haven't thought of would be good to.
I need the GPIO pins that are not set to HIGH to not become a ground connection.
No you don't. There always needs to be something between your motor and the GPIO, as the GPIO cannot handle motor currents, and in your case either you want a unipolar motor driver or your L293 and a different stepper motor.
If you do use your L293, you don't need to supply a high impedance to it.
If you only drive one coil at a time, you should be able to ignore the common connection. Alternatively, you might be able to connect common to one of the four half bridges, and two of the other four coils to the two opposite sides (I don't currently have time to work out whether this would give you all the states required but I think it will as L293 allows setting each half bridge to be the same or different to each other).