I suppose I should start the post by describing what a gerund is, in order for us all to be on the same page. In extremely non-technical terms, a gerund is a word-type that looks like a verb with an -ing suffix, but also looks like a noun because it can be the subject of a verb. For example, the following sentences contain gerunds (bolded):
Eating is enjoyable.
Reading Chomsky makes my head spin.
I can’t believe Mark’s cheating on the test doesn’t bother you.
These are tricky words to describe, and traditional grammars are often at odds with each other about how to treat them. Some people will tell you these are definitely verbs, but others will insist they are nouns. Some grammarians take a half-way view and say something like “gerunds are verbs that act like nouns”.
It turns out that this is one occasion where I will not be picking apart the traditional view, because things really are messy. Gerunds have weird mixed behaviour that makes them look a little bit like verbs, a little bit like nouns. They also have unique properties of their own that they don’t share with either nouns or verbs.