In addition, Gallen will be ineligible to be selected for the Australian Kangaroos in 2015 unless he successfully completes a leader’s accountability course. His eligibility will ultimately be determined by an ARLC panel.
CEO Dave Smith, said the tweet on social media was offensive and a clear breach of the Code of Conduct.
It was aggravated by the fact that Gallen, as Captain of the NSW Blues, holds an important leadership position in the game.
A breach notice has been issued to Gallen, giving him seven days to state why the fine should not be imposed.
He will be free to put forward any mitigating factors as part of his response.
Mr Smith said the $50,000 fine was the maximum the NRL could impose on a player for a breach of the Code of Conduct.
It had been determined in collaboration with the Cronulla Sharks.
Cronulla Sharks Chairman Damian Keogh said the club supported the strong sanctions imposed on Gallen.
“The last two years have been a difficult period for the Sharks and our focus is on performing strongly on and off the field in 2015,” he said.
“So it is disappointing to have the club involved in this sort of issue as we prepare for next season.”
Mr Smith said the NRL would also discuss the NSW captaincy issue with the NSWRL which is responsible for Origin selection.
“Origin is one of our flagship properties and we do not want it to be undermined in any way,” he said.
The NRL will not make any further comment on the matter until Gallen has had a chance to respond to the breach notice.