Translation:Your nice smile is very effective for flirting.
I don't understand why they are trying so hard to redefine ĝentila as "nice" in this section.
ĝentila - well-mannered, polite, courteous (https://lernu.net/en/vortaro/)
ĝentil/a. Obeanta la regulojn de la socia deco; observanta la bonajn manierojn (http://vortaro.net/#%C4%9Dentila)
agrabla - nice, pleasant, agreeable, enjoyable
por flirti and por flirtado are basically interchangeable.
Note that "flirti" means to flap in the breeze like a flag or to flutter from flower to flower like a butterfly. It implies "playing the field" in a way that "to flirt" does not. A more neutral word for courting someone or trying to get the attention of someone you like is amindumi.