Gannicus' short backstory upon returning to Capua hints at his fighting as gladiator to make his living, despite being free from slavery. Historically speaking, roughly 40% of the gladiators who fought in Rome were free men as the trade was not quite as dangerous as the show portrays.
In the past, Gannicus could always be seen smiling upon entering the arena. However this time, his face is solemn and serious.