If the head gasket is replaced, should I do the timing belt and water pump?

The engine will be out to replace the head gaskets on that engine there is no additional labor to replace the timing belt it is off any way, additional labor to replace w/pump is only ten minutes.

If the timing belt hasn't been replaced before, it would be highly recommended to do so during the HG replacement. Gates makes a full Subaru Timing kit that includes new tensioner, water pump, belt and idlers. If skipped at this time, timing belt failure can lead to much more expensive repairs.

The timing belt is over due unless you changed at 100,000 miles. Always change the small stuff while your in there. You will avoid having to do it sooner than later. Most kits come with the water pump and gasket. Think about doing the thermostat while you have the anti-freeze out. there is likely a new thermostat gasket in the kit.