The gerund always has the same function as a noun.
A gerund can be the subject in the sentence. Examples:

• Walking is good for you.

A gerund can be an object in the sentence. Examples:

• I like walking in the park.

A gerund after prepositions
After prepositions we normally use a gerund. Examples:

• Can you sneeze without opening your mouth?
• She is good at painting.
• I was the fastest at climbing the rope.
• What about going out tonight?

A gerund after certain specific verbs. Examples:

• I like cooking.
• He enjoys walking.
• They hate milking cows.
• I can imagine drifting away in a balloon.

A gerund in compound nouns. Examples:

• I took her to her driving lessons.
• We are going to the swimming pool.
• I am good at rope climbing.