Take a look at the following example:
We’re trying to come up with some completely fresh thinking on this subject.
“Thinking” must be singular, the sentence is correct and means not just “new ideas/thoughts” but also “a new approach” (approach is more active than ideas).
For the below example, you could use either the gerund or the noun, depending on what was important in your intended meaning.
The sale of the car was finalized last Friday. (not the action of selling, but the legal agreement, which is a thing, so you use the noun)
Selling the car was easy because it was in great condition. (the process/action of selling is what is important, so you use the gerund)
The sale of the car was easy, because the buyer was in a hurry and he didn’t even read all the fine print in the contract. (not the active process of selling, but the sale agreement, which is a thing)
To summarize, it’s a question of nuance. When action/process is important to your meaning, you are more likely to use the gerund form. When the action/process is not important, you are more likely to use the noun.