Hi Allen -
That will depend on the drum kit (software instrument) you use. Most of the GB kits use the General MIDI drum map, so if you play A#1 (open hihat) followed by G#1 (hihat foot), the G#1 key will close the open hihat (like a drummer stepping on the hihat pedal). The F#1 key (closed hihat) will also close the A#1 open hihat. Some drum kits have more elaborate hihat mapping -- for example, some of the Drummer kits use the modwheel to open & close the hihats -- but the F#1/G#1/A#1 layout is the most common implementation.