Complex sentence: In 2000, Gillingham reached the second tier of the English league for the first time in the club's history and went on to spend five seasons at this level, achieving a club record highest league finish of eleventh place in the 2002--03 season.
Split into simpler sentences: In 2000, Gillingham reached the second tier of the English league for the first time in the club's history. They went on to spend five seasons at this level, achieving a club record, highest league finish of eleventh place in the 2002--03 season.