The topless bronze gladiatrix in Hamburg.

photo Alfonso Manas

Female gladiators existed!
A rarity, nevertheless, documented by Roman historians such as Dio Cassius (150-235 CE) who describes a festival organized by Nero for his mother (in which women fought). 
The same writer reports an event sponsored by the same emperor in the year 66 CE with Ethiopian women-gladiators in Pozzuoli in honor of the king of Armenia Tiridates.
Suetonius, another famous Roman historian and biographer (69-122 CE), describes games held under Domitian (88 CE) in which women fought against dwarfs!
Tacitus (Annales 15.32-33) reports that during a show in AD 63 some senators and noblewomen entered the arena to fight.
The satirical poet Juvenal describes Mevia hunting boars in the arena. 
Petronius (Satyricon) and Martial (Liber De Spectaculis) also refer to games involving women.
In the British Museum a marble relief found in Halicarnassus (Bodrum, Turkey) portrays 2 women gladiators nicknamed Achillia and Amazon. 

A bronze statuette, almost 2000 years old, in the Museum für Kunst und Gewerbein, in Hamburg,  has recently been interpreted as woman-gladiator, according to researcher Alfonso Manas, of Spain's University of Granada.  It could be a bare-chested woman brandishing a short, curved sword (sica).  The mysterious object she holds was previously identified as a strigil (used by Romans, who did not use soap, for scraping the body clean).
Manas is persuaded she is a gladiator for the pose:  typical of victorious gladiators.
Not surprising that she is topless:  women and men fought with bare chest.
Considering the audience was largely formed by men that caused a certain impact of course.

The scarcity of archaeological evidence depicting women gladiators however proves that women were rarely fighting. Such custom was banned by the emperor Septimus Severus (200 AD).