I’m sorry but the staff killed Gemma. I think all the scenes where we see them eyeing each other is them plotting to kill her.

Although Kate hated her, I really don’t think she killed her. There’s a definite theme through the show of “all of you hate each other” so if that’s the only reason people think Kate killed her, it isn’t strong evidence.

I also think Kate holding the knife is a purposeful red herring. It fits the whodunit structure (always a red herring somewhere) and the killer being a staff member fits the “it’s always who you least suspect” trope.

I also think the staff would have a much easier time committing the murder as their movements would be less scrutinized. Also since they play this game every year, I’m sure they would know the perfect time and place to kill her. I suppose Kate would also know, but again I think her hatred would not manifest as homicidal.

There is also no trophy taken. You could say maybe Joe interrupted before one could be taken by Kate, but I think it’s unlikely.