Sorry the use of GPIO objects in "real" patches isn't clear from the book. I'll tackle that one in the next revision of the book.
Here's a quick exlplanation:
The gpio/in/analog objects will give you a signal from 0 to 64. If you want to use this signal to control values between -64 and 64, you need to convert the signal with a conv/unipolar2bipolar object.
So if we take a look at the filter/vcf3 object, you'll notice that there are two blue inlets, one for the pitch and one for the resonance of the filter. The pitch dial goes from -64 to 64, so you'll need to convert the incoming signal. The dial for the resonance is 0 to 64, so there's no need for conversion. If you leave the dials at their default position, you can control the full filter with the potentiometers.
Some inlets/outlets on objects will also feature a + or ± sign to show more clearly what signal is expected.
Hope this helps you.