The 34-year-old has previously worked in the Australian A-League and has spent much of his time in England officiating matches in the lower leagues.
During his time in Australia, Gillett took charge of five A-League grand finals and was also named the competition's referee of the year on five occasions.
Gillett has also been the VAR for past Premier League matches as well as a fourth official but will make history when he leads out the teams at Vicarage Road this weekend.
The Australian was involved in the Championship match between Peterborough United and Birmingham City on Saturday where a player made a complaint to him over racist abuse from supporters.
Gillett was also involved in a controversial decision as a VAR during Brighton's win at Burnley on the opening weekend of the Premier League campaign.
Gillett, who also has experience as a FIFA international official, was promoted in June to the top Select Group allowed to referee in the Premier League.