The gerundio can be used just in adverbial expressions:

Sonriendo me dijo que me odia. Ladrando los perros cruzaron la calle. Diciendo que me quiere mucho me dió un beso. Hablando se emborracharon.

Smiling she told me that she hates me. Barking the dogs were running across the street. Telling me that she loves me she gave me a kiss. (The told me she loves me and gave me a kiss) Talking they were getting drunk. (They were talking and getting drunk)

REMEMBER! Has the English gerund an adjectival meaning then you cannot use the Spanish gerundio, but you need to describe it with a sentence.

Sleeping men are happy.   Barking dogs are disturbing.   The smiling woman is beautiful.

= correct: Los hombres que duermen son felices.    incorrect: Los hombres durmiendo son felices. = correct: Perros que ladran molestan.    incorrect: Perros ladrando molestan. = correct: La mujer que sonríe es guapa.    incorrect: La mujer sonriendo es guapa.

Actually it's quite logical because the adjective has to be in gender and number in accordance with the subject. However, gerundios are unchangeable.