Isn't a “gonner” or “gonna” slang for a person about to die?

It is spelled with one 'n' because it comes from "gone" (not from "gonna" - going to) as in earlier expressions like gone goose or gone coon.

Goner (n.):

• "something dead or about to die, person past recovery, one who is done for in any way," 1836, American English colloquial, from gone + -er (1). From earlier expressions such as gone goose (1830), gone coon, etc. (Etynomline)

• According to Ngram the double n variant is much less common but probably as old as goner.

Gonner is a rare spelling variant of goner.

Goner

TFD n. slang One that is ruined or doomed.
"I'm a goner if this plan doesn't work"

M-W n. someone or something that is going to die or that can no longer be used
"This old computer is a goner. We'll have to get a new one. "

Wiktionary etymology: gone +‎ -er