Answer:
It depends on the gladiator in question. Different types wore different armor, therefore they used different techniques to target different parts of the body. Common places of injury however would have been directly behind the knees, as Gladiators often used their dagger/sword hand to reach behind the leg, pulling it up and pushing the opponent off balance, or severing the hamstrings in the least. Another place a gladiator would have suffered injury would have been the back, as they would have tried to reach over the back in an attempt to access the un-armored section. They probably would have often suffered broken noses, from getting struck in the face with the shield. The side would have been an easier target than the chest, especially with the curved Thraex dagger. The thighs would have been a popular place to aim as well.
For further information, tom_mrsn@yahoo.com.