Some answers in no particular order:
1. There are lots of alien teams as well which we will get to but humans are particularly savage and warlike.
2. Human uprisings - you'll have to wait to hear about that one 😜
3. The players are VERY aware of what they've been put into - depending on who they are they have different attitudes to their status (basically slaves although some owners treat them very differently than others)
4. I don't know what "Gunner Games" is but arenas come in all shapes and sizes and are configured differently for different themes and matches. It's a big show. Some arenas are better known than others. Some smaller/less important worlds have more simple set ups.
5. Games take a variety of forms - some are just head to head matches with a standard field configuration so that tournaments can be held, others are narrative. There is every type of objective/mission you can think of. The physical environment (gravity, air, dangerous flora and fauna) can all be alterred/configured.
6. Matches are held for both private audiences and broadcasts. Not all galactic citizens support the sport and consider it barbaric. Nevertheless it has a massive following
7. Players are injured and "die" on the regular but advanced medical technology, cloning, and consciousness backups allow players to come back to the field. In some cases owners are more generous and allow players to have families (some of which go onto be players themselves) and/or walk away from the games once new players are trained up. In other cases, owners wring every last bit of performance and "value" out of their slaves. It is those owners whose players are more likely to end up as Cannon Fodder as they are described: "These are the dregs of the Death Fields sport: the washouts, the mental cases, the warriors who just can't cut it or the teams whose strategy, training, or cunning have fallen behind new, younger franchises... These are also the one final chance for an owner to profit."
8. Earth and it's stellar colonies are in the backwater of the galaxy. They have not advanced to a point where they would be invited to join the larger civilization. Distance and deliberate subterfuge keep humans from getting too close to the core system. But as they expand, contact is becoming more likely.