Use the wheel or drag the mouse (for knobs use up/down) to make adjustments. Press ctrl or shift key while dragging to make fine adjustments. Double clicking sets the control to the mouse position.
Click on a value display (or click the control with the right mouse button) to open a small popup window for value entry. Use enter or ESC key to close the window.
Clicking on a list selector sets the next value, right button click displays a popup list. The wheel can also be used on switches and list selectors.
Most controls can be connected to MIDI gear by clicking with the middle mouse button.
When a control has the keyboard focus (e.g. after clicking on it) you can use cursor keys, PgUp, PgDn, Home, End to change the value (but in the current Guitarix UI the focus widget is not visually distinguished).