The GCLK pin is a pin connected to a global line inside the device and is suitable for assigning flip-flop control signals such as clock and clear. This pin can be used to de-skew the control signal fan-out path.
 
The MAX II can also be used as a user I/O pin, but when not in use, treat it the same way you would treat an unused user I/O pin.

Also, the GCLK pins of other MAX devices are input-only pins and should be tied to GND (or VCC) on the board if unused.