It was not a question of them being able to get a car.
Yes. It's a gerund. Its subject is them.
In English, sometimes it's hard or perhaps impossible to say whether the -ing form of a verb is a present participle or a gerund. In this sentence, one could argue that being is a participle modifying them. But I think it's clear that the question is about whether "they" could get the car, not simply about "them".
There is also another way to indicate the subject of a gerund, and that's to put the subject into the possessive case:
It was not a question of their being able to get a car.
The whole clause, whether you word it "them being able to get a car" or "their being able to get a car", is the object of the preposition of. So, the clause is functioning as a big noun. This is the main thing gerunds do: they enable you to package up a verb, complete with its own subject and predicate, and treat it as a noun in a larger sentence.
"A question of them being able to get a car" is the subject-complement of "It".
The main verb is "was". The main subject is "It".