A gerund is one of several verb forms, called verbals. Verbals include gerunds, present participles, past participles, and infinitives. A gerund is a verbal that ends in -ing, just like a present participle. The difference between those two is based on function, rather than form. A gerund is used as a noun. It turns an action into a concept, so we can discuss it as a thing.
I like swimming more than jogging.
In this sentence, the verb is "like". The writer is expressing a preference for one activity over another. Both -ing words are gerund-nouns. They are not functioning as verbs in the sentence. They have become things similar to: I like pizza more than vegetables.
The point of confusion with gerunds is being able to tell them from present participles, which, unfortunately, have exactly the same form. Present participles are used in the formation of progressive verbs and are used as adjectives to modify nouns.
Last week, I was swimming in the ocean. [was swimming = past progressive verb]
At the circus, I saw a dancing bear. [dancing = adjective, describing bear]