The incident took place on Tuesday when, during the post-third day's play media briefing, Gibson challenged the DRS and its implementation by the match officials.
He was subsequently charged with breaching the ICC Code of Conduct.
"In the pre-series meetings it was agreed that players and coaches should not engage in negative media comments. Ottis accepts that he overstepped the mark during the game and has now accepted his sanction," said Jeff Crowe of the Elite Panel of ICC Match Referees.
Gibson had been frustrated by what he saw as inconsistencies in the system caused by the absence of Hot-Spot and Snicko technology.
"I can't say what I really want to say about the DRS because the ICC will sack me or ban me or whatever," Gibson had said.
"If the ICC is going to use DRS I think they should use all the technology and I think if we haven't got all the technology we shouldn't use it at all."
Australia won the first Test in Barbados before the rain-affected second Test here ended in a predictable draw.
The final match starts in Dominica on Monday.
