Hi Pedro,
The winner of a gladiator fight would be awarded a palm leave. There are some nice photo-etched or laser-cut paper leaves out there.
I have the book by Markus Junkelmann about gladiators and will have a look if I can find something for you. I once depicted some flowers thrown to a gladiator, as I'm pretty sure this is of all ages, see my vBench for the figure.
Be careful with using paintings such as the one above for reference. They are usually from the 19th century, when there was a great revival of interest in the classical world, but the knowledge of it was rather inaccurate. For example, the winning gladiator in the painting above has a fish on its helmet as this relates to the name of this gladiator type. This has been shown to be inaccurate by now. No such helmet was ever found for example. So is the thumb movement. Current understanding says there was no such thumbs up or down gesture. There were gestures, but I'd have to look up how they were again....
I do love the painting nevertheless and think it is really well done. Excellent NMM for example!
Cheers,
Adrian