A total of 692 eggs were used in the experiment. Thirty-two embryo clones were initially implanted in surrogate mothers, but only eight of the adult cows became pregnant.
The Asian gaur should be delivered by its surrogate mother in November.
Some researchers believe cloning may offer the only way to save species from extinction. But conservationists argue there is little point if the reasons for the animals' decline, such as habitat loss, have not been addressed first.
Giant panda
Researchers at Advanced Cell Technology (ACT) in Massachusetts fused the genetic material from a dead male gaur with the empty egg cell of a cow to make their clone, which they have already called Noah.
A total of 692 eggs were used in the experiment. Thirty-two embryo clones were initially implanted in surrogate mothers, but only eight of the adult cows became pregnant.
Of those, five miscarried, and two other foetuses were aborted for scientific purposes. A cow called Bessie now carries the one remaining gaur foetus.
"Noah will be just the first creature up the ramp of the ark of endangered species that we and other scientists are currently attempting to clone," researchers Robert Lanza, Betsy Dresser and Philip Damiani wrote in an article about their work that appears in the magazine Scientific American.
"Plans are under way to clone the African bongo antelope, the Sumatran tiger and that favourite of zoo lovers, the reluctant-to-reproduce giant panda," they added.
Both sexes
ACT also intends to revive an extinct species of Spanish mountain goat called bucardo.
The last known bucardo was killed nine months ago by a falling tree. But some of the animal's cells were preserved and these will now be used to bring the goat "back to life" next year.
Normally, a clone shares all the genetic characteristics of the original cell, including sex, which means a breeding population cannot be created.
But the ACT team hopes to gain permission from the Spanish authorities to use the latest molecular techniques to insert male chromosomes from a closely related goat species, creating male as well as female bucardos.
However, some observers have deep reservations about the use of cloning to protect endangered animals, or even retrieve extinct creatures from history.
Degraded habitats
Dr Amanda Pickard, of the Zoological Society of London, said: "Cloning will only be able to generate a small number of individuals and that does not make a viable population in the long run.
"We need to think of more basic genetic management techniques which are going to make a population viable for a longer period."
And conservationists say there is little point in hanging on to animals if habitat loss means the only place the creatures can live is in a zoo.
Gaur numbers have dwindled to about 36,000 because the animals have been hunted by humans and because the forests, bamboo jungles and grasslands in India and Southeast Asia where they live have become degraded.
'Fighting chance'
But the ACT team said it was simply not acceptable to sit back and watch animals disappear.
"I've heard a lot of people saying we are playing God," Dr Robert Lanza told the BBC. "Well, we do play God when we wreak havoc on the environment, we play God when we destroy their habitats and shoot them for sport.
"The least we can do is try to reverse some of that damage, to give these species a fighting chance of surviving in the wild."
It is thought about 11% of birds, 25% of mammals and 34% of fish have numbers so low that their survival is threatened.