Ganaste is the conjugation of the verb ganar that matches the second person singular (informal form) in the preterite tense. So the subject pronoun that matches this person is tú. Furthermore, the preterite is a past tense we use to talk about something that happened at a certain moment in the past. Finally, we can write this past sentence as follows:
Marta: Tú ganaste 6.000 pesos.
