Who else could have aided Gondor?

[Gurthang]

"Only from Rohan now will any men ride to [Gondor] when we call"

He's seems to be saying that the ones that did answer the call before would not now. I don't know of any others Gondor has asked for help other than Arnor and the Rohirrim. In the case of the horse-lords, they came after hearing a general plea for help. Perhaps Denethor had sent out a similar plea, but this time no one came.

The only possible answer could have been from someone in Rohvanion, or the Vale of Anduin. There aren't that many places that men still dwell.