Mr Gethin succeeds Tom Baxter-Wright who has retired as Chairman after many years of outstanding service to the Trust.
"I'm delighted and honoured to take up the position of Chairman for the Welsh Rugby Charitable Trust which is dedicated to improving the well-being of severely injured rugby players," said the new WRCT Chairman, who first joined the charity on his appointment as President of the WRU in 2007.
"Having had the privilege of meeting a number of these brave and inspirational individuals, I am looking forward to my term as Chairman and will do everything to support these players and make a positive difference to their lives and the lives of their families.
"I wish to thank Tom Baxter-Wright for his extremely valuable contribution. I know that all his hard work is greatly appreciated by the players and their families and he has carried out his duties with dignity and professionalism."
The WRCT has also appointed two new trustees, former Wales scrum half Rupert Moon and journalist and writer Carolyn Hitt.
Mr Gethin added: "The Trust brings together the enthusiasm and experience of some fantastic individuals to ensure that the injured players are represented in the best way possible and I know that Rupert and Carolyn will be a great asset to the Trust which will benefit from their undoubted ability.
"It is also a particular honour to follow in the footsteps of the late Sir Tasker Watkins who was a founder member and Chairman of the Trust. I would like to thank the Trustees for their confidence in me, and a special thank you to the Honorary Secretary Edward Jones whose work for the Trust is truly remarkable.
"Finally I would like to pay tribute to the Welsh Rugby Union for their invaluable help both financially and in many other ways, and to the many rugby clubs and organisations throughout Wales who continue to lend their support."