The G-Force can handle up to 8 external modifiers. Of these one can be connected through the expression pedal socket, the rest (or all) must be connected through MIDI. This is done in the I/O Setup.
An expression pedal can be used as an external modifier through the expression pedal jack (at the back of the G-Force) or through a MIDI board. To calibrate the G-Force's global response curve, go to the Utility menu and follow the instructions (version 1.12 and higher). You can also calibrate a pedal for individual patches by changing the associated Link response curve.
Design of expression pedals
Usually the electronic components inside a volume- or expression pedal consist of a potentiometer (pot), with perhaps a resistor or two for calibration purposes and a capacitor to avoid high frequency loss at low levels. There may also be an additional potentiometer for calibration. At least some wah wah pedals and one MIDI board (ART X-15) use optical sensors, but I don't know if they are used in any expression pedals. Unlike potentiometers optical sensors need power in order to operate, on the other hand they don't oxidize or wear out over time like a potentiometer will do.
The difference between volume- and expression pedals
While volume pedals have input and output jacks and are inserted in the audio signal chain (say between guitar and amp), expression pedals work different. Here the effect unit sends out a fixed voltage (about 5 volts) through its expression pedal jack, and then measures how much of this voltage the expression pedal returns. The returned voltage is used to control the effect's level. Some expression pedal jacks on effect processors are intended for TRS (Tip/Ring/Sleeve or Stereo) plugs, while it seems others are made for TS (Tip/Sleeve or Mono) plugs (see the page on audio electronics for more information about plugs).
Use expression pedals to morph between sounds (Greg Scott gets credit for this tip)
By linking one or two expression pedals to several effect parameters it is possible to morph (change gradually) between 2-3 different sounds in one preset. You can also use an expression pedal to pseudo-bypass effects by changing their Mix values between 0-100%.
Expression pedals are not the only kind of external modifier. Below are a few other suggestions, even more MIDI controllers can be found at synthzone.com.
MIDI switches will move the marker in the G-Force's Link response curve instantly from one side to the other, so if you only move a pedal between heel and toe position anyway a switch might be a faster alternative.
Breath controllers may not be manufactured anymore, but where made in the 1980s for controlling MIDI gear. When you blow into the breath controller the air flow is translated to a MIDI signal. Allan Holdsworth used one for making volume swells on a synth guitar, but similar effects should be quite possible on the G-Force as well. You could also use a breath controller with a Wah effect.
Sequencers can be used to modify effect parameters or changing presets, making a foot controller unnecessary. When using a sequencer like this you probably also want to connect it to other gear, such as a synth or drum machine, so you know where in the sequencer program you are.
A keyboard player might modify G-Force effect parameters (using the keyboard's Pitch bend, Velocity, Level or Aftertouch effects).
A drum machine's internal sequencer may also be used to control G-Force effects (see above). This could be useful to create very tight intros or endings, if you link the drum machine's sequence to say a G-Force level parameter. Unfortunately drum machines cannot have their tempo synchronized with the G-Force tempo, so they must be programmed separately.
Electronic drum pads can be used to control a G-Force effect level: the guitar player strikes a chord that stays muted until the drummer trigs it. Perhaps MIDI drum pads can also be used to modify ordinary effect parameter, like having a drum roll control a G-Force Filter or Panner sweep.
MIDI faders are devices made especically for controlling MIDI gear. With these you can access eight G-Force parameters simultanenously. Since modifier positions cannot be saved into presets you must either save them in the external device or use the tip about saving modifier settings.
Digital mixers might be used as MIDI faders or as sequencers, where the varying mixer settings (used as G-Force Modifier values) will be replayed during the sequence.
MIDI guitar pickups could be used as possibly great external modifiers. Normally a MIDI pickup analyzes the strings' pitches and dynamics, this information (not any audio signal) is then sent as a MIDI instruction to a synth, that in turn creates a note from it. But when used as a G-Force modifier the MIDI instruction is not sent to any synth, instead it's used to modify the G-Force's effect parameters. The traditional guitar pickup signal is sent through the ordinary preamp and G-Force audio input as usual. MIDI guitar pickups can be used to
make an improved Pitch Detector, where you can let the pitch of just one guitar string be used as a modifier
let individual strings have their own effect parameter values.