Were the Elves allies to Gondor?

[Ñólendil]

I mean if the elves had stayed in Middle-Earth into the Fourth Age and Gondor was under attack by a force greater than that on the Pelennor Fields the surely the elves would have ridden if only to just save Arwen. I can't imagine Elrond, Elladan and Elrohir just watching Gondor and their daughter/sister be destroyed.

Many Elves did stay in the Fourth Age, but that aside, this is such a hypothetical question it is hard to imagine the answer. It would depend on the circumstances, ... but if Rivendell did not send an army in the War of the Ring, why would they send one in the Fourth Age? You suggest they would go to "save Arwen", but that would suggest the only reason Rivendell did not help much in the War of the Rings because they're own people were not in immediate danger in the land of Gondor. I disagree with this--Rivendell did not send an army for a few different reasons, but one of them would have to be that sending enough to make a difference would deplete their own defenses, and leave no one left to defend or flee should the tide of War sweep into Eriador. Anyway, in the Fourth Age Arnor was rebuilt, and the Reunited Kingdom of Arnor and Gondor was a mighty one. Any army large enough to crush it would not be bothered by a force of Imladris.

Gondor and Rohan actually did go to war in the far East (and possible South, I don't remember), many times, before Elessar passed away.

But what are you driving at?