The histories are vague when it comes to Spartacus' gladiatorial origins. Appian mentions that Spartacus was a Thracian who served with the Roman army, most likely as an auxiliary force. Florus gives more detail saying "[Spartacus] being a Thracian mercenary, had become a soldier, and from a soldier a deserter, then a highwayman, and finally, thanks to his strength, a gladiator." (Florus, Epitome, II)

So it is implied that Spartacus started as Thracian warrior, perhaps auxiliary, and somehow landed himself on the wrong side of the law and became a gladiator. To me this implies, Spartacus became a gladiator by force. However, as I mentioned, Spartacus' early years are veiled in mystery as he only became of note once he started the Third Servile War.