Usas "mucho gusto en conocerla" cuando es mujer, "mucho gusto en conocerlo" cuando es hombre. Puedes usar "mucho gusto en conocerle" en cualquiera de los dos casos, pero en España se usa "conocerle". Además se usa cuando conoces a una persona de más edad o una autoridad política o de alguna empresa (lenguaje formal). En el caso de "mucho gusto en conocerte", se usa cuando conoces a una persona de edad similar a la tuya y se usa si es hombre o mujer (lenguaje informal). Espero que te haya servido.

As Alberto said, if the person is much older than you, and you are using usted, definitely I would say, mucho gusto en conocerla for a female, and for a male, mucho gusto en conocerlo, for a male. What I wanted to add that indeed, in Spain they would use conocerle for a male, but this is known as leismo, and many grammar professors would tell you this is against the logic of the Spanish language, and many Spaniards aren't even aware of this grammatical rule. So i would stick to "lo" even if talking to a Spaniard because gramatically speaking, you won't be wrong. If you want to learn about leísmo, i pasted the simple wikipedia about it below. For a spanish article on why some consider it incorrect, you can amuse yourself here: http://bit.ly/1SztL8z Leísmo ("using le") is a dialectal variation in the Spanish language that occurs largely in Spain. It involves using the indirect object pronoun le in place of the (standard) masculine direct object pronoun lo, especially when the direct object refers to a male person. Leísmo with animate objects is both common and prescriptively accepted in many dialects spoken in Spain, but uncommon in most others. Leísmo is always rejected in linguistic prescription when the direct object to which it refers is not an animate object. Examples: Veo al chico ("I see the boy") → Lo veo (standard Spanish, with lo) Veo al chico ("I see the boy") → Le veo (leísmo, common in Spain; other regions prefer lo veo)

People already said it but a summary: "conocerlo" if the person is man. "conocerla" if the person is a woman. "conocerle" is used in Spain and can be used with both women and men.

Se dice "Gusto en conocerte"