Don't confuse gerunds with present participles, both end in -ing but serve different purposes. I am swimming (present participle), versus Swimming is relaxing (gerund).
Pro Tip 2/3
Not Always Actions
While gerunds are verb-based, they don't always describe an action. Sometimes they name activities or ideas. In 'Thinking can be hard,' 'thinking' refers to an idea, not an action.
Pro Tip 3/3
Versatility
Gerunds can act as subjects, objects, or complements, giving them versatile roles in sentences. Eating is necessary (subject), I enjoy eating (object).