As a native speaker, but not a grammarian, I'm puzzled. I can give usage examples but not explain them. I would say that when a word is truly a gerund--manufactured out of a verb by adding "-ing"--then no, it is not countable and does not have a plural form. Or not usually.
Skipping, testing, thinking, listening. They have no plurals.
But either there are many exceptions, or there are many words ending in "-ing" with a verb-like root that are not gerunds, or there are a lot of gerunds that have gotten a special status as nouns.
I can say "he is writing" or "he and she are writing." But I can speak of "Charles Dickens' writings."
I can say "He is baking zwieback," or "Zwieback is baked twice, and each of the two bakings is done on a different day."
In short, I don't know.