The bird is part of a pair of birds which the entrepreneur had specially imported for the lake of his Raphoe mansion.
But one of the pair, which are supposed to be mate for life, has mysteriously vanished from the lake at plush Oakfield Park.
The bird, which had its wings clipped so it could not fly away, went missing last Saturday.
A spokesman for Oakfield Park said they did not know what had happened to the bird which is tame and eats out of people’s hands.
They are hoping it may be in nearby fields and unable to find its way back to the lake.
The bird is not native to Ireland and comes from Australia.
The spokesperson added that former Granada TV boss Gerry is particularly saddened at the loss of the rare bird.
“The swans are here about eight months and we were hoping they eventually would nest here.
“It was a very friendly bird and would come up to people visiting here on tours and eat out of their hands.
“We don’t know how it happened but it may have grown its feathers and simply flown away.
“Sir Gerry really liked looking at the two birds out on the lake,” he said.