What is a predicate nominative when you are talking about gerunds?

see below

A gerund is a verb form (ing-form) that acts as a noun. It functions the same way as ordinary do:

• as subject
• as direct object
• as indirect object
• predicate nominative

Below are examples of sentences using gerund as predicate nominative:

1. Michael's favorite sport is kayaking . "Kayaking" renames the sport.
2. Her hobby is collecting stamps . "Collecting stamps" is a gerund phrase that renames hobby.
3. Marie's priority is taking care of her family . "Taking care of her family" is a gerund phrase that renames priority.
4. His favorite time is strolling . "Strolling" is a gerund that renames past time.