Converting Eurorack's -5/5 V to Raspberry Pi's 0/3.3 V
First, there's no ADC on the Raspberry Pi, so you can't just read analog values with Pi even if you scale the voltage range to acceptable limits. All GPIO terminals on the Pi are digital. 

If you just want to receive digital signals it's super easy to do with just a transistor. Here on this schematic the R2 and Q1 form a voltage divider. The center point of the divider is connected to GPIO of Pi. 
When your input voltage is somewhere above 0 (so enough current is flowing into the base of Q1) the transistor will open so voltage between collector and emitter will become low. GPIO level would be low (0).
When input voltage is low or negative the transistor will close and it's resistance will be much higher than R1 so GPIO level will be high (1).
Effectively it is an inverting input, I think it shouldn't be a problem for your software.  







simulate this circuit &ndash; Schematic created using CircuitLab

You would have to connect 3 wires to your Pi GPIO header: +3.3V, GND, and one IO pin (input). 
And don't forget that input is two wires: the input signal and the ground connection.

The Q1 part number 2N3904 is just an example. Any small signal NPN transistor will work here.