In the sentence "Talking too much is my chief problem." The gerund is Talking, and its noun use is subject.
Gerund is a word ending in ing that is made from a verb and used like a noun, and has the same form as the present participle (working, speaking etc.). I can be used as subject of a sentence ( Reading French is difficult.), as complement of verb (His hobby is painting.) etc. Gerund functions like a noun.
