(I think this "blank" moment of mine is what is called in AmEng a brain fart, so be it)
Isn't ‘a gonner/gonna’ slang for a person who is about to die? It's said in situations where, potentially, someone risks getting themselves killed. Maybe children also use it when play acting. I'm sure there must be something similar but the dictionary entries tell me that ‘gonna’ is a contraction of "going to" and ‘gonner’ is not listed in The Free Dictionary.
If you do that (a dangerous thing) you're a gonner
I presume a gonna or a gonner (or something similar) is short for “a person who is going to die”.