Gabriel's remarks were in breach of the ICC Code of Conduct, for which he was also fined 75 percent of his match fees.
Gabriel, who received three demerit points, had accumulated a total of eight demerit points during a specified period of 24 months, the ICC stated.
"During the third day of the St Lucia Test against England on Monday, Gabriel was found guilty of breaching article 2.13 of the ICC Code of Conduct, which relates to "personal abuse of a Player, Player Support Personnel, Umpire or Match Referee during an international match," said the ICC in a statement.
While what exactly Gabriel said wasn't audible, but according to a report, the England captain Root was heard saying, "Don't use it as an insult. There's nothing wrong with being gay."
Gabriel confesses the offence after the end of the match on Tuesday and accepted the sanction proposed by match referee Jeff Crowe and no formal hearing took place.
On-field umpires Rod Tucker and Kumar Dharmasena and third umpire Chris Gaffaney levelled the charges.