Why do Gurkha's fight for the British army?

Why do the gurkhas fight for the british army. At what time in history did this occure? Why do the gurkhas fight for the british army. At what time in history did this occure? Why do the gurkhas fight for the british army. At what time in history did this occure? Why do the gurkhas fight for the british army. At what time in history did this occure?