I think some often-overlooked context for gideon’s sacrifice at the end of GTN is that, if gideon survives, harrow asked her to return to drearburh.

of course, harrow’s plan of I’ll-hold-the-lyctor-off-you-and-cam-jump-into-the-sea is dogshit, not going to work etc. of course gideon is a perpetual Good Girl, a butch in shining armor–she’s a saint. she’s jesus christ. I don’t mean at all to undermine her selflessness, but in her panic i can’t help but think the threat of the ninth house made the choice much simpler.

let’s go back to the beginning of GTN to the amalgam of everybody’s worst hometown:

"she was takne as a very small bondswoman, not of the Ninth but beholden to it: What greater debt could be accrued than that of being brought up?"ALT
"Gideon's appeals to better natures, financial rewards, moral obligations, outlined plans, and simple attempts to run away numbered eighty-six by the time she was eighteen. She'd started when she was four."ALT

gideon the ninth, chapter 1

gideon’s entire life has been a series of escape attempts. in canaan house her relationship to harrow undergoes a metamorphosis, but behind her is eighteen years of trying–relentlessly–to run from a cold dark place that she is beholden to. (whenever beholden comes up in this series at least 8 alarms go off in my head and i drop whatever i’m holding).

"There was something urgent in her voice: something worried, something new. "Things are changing. I used to think we were waiting for something ... and now I think we're just waiting to die." Gideon's heart sagged. "You really want me to say yes." "Go on and say no," said her captain. "It's your choice ... If she doesn't take you, I'll go with her and gladly. But she knows ... and I know .. and I think you damn well know ... that if you don't get out now, you won't even get out in a box."ALT

gideon the ninth, chapter 4

I think aiglamene understands drearburh is inherently soul-killing in a way that harrow does not. harrow genuinely loves her house (which, to her, is the Tomb). gideon was never allowed to love the ninth house–it rejected her at every opportunity. harrow, however, threw herself into it wholesale. she had nothing else.

skip to the pool scene. harrow receives absolution and a emotionally charged tender forehead kiss (top 10 lesbian baptisms of all time). they are finally on the same side, out from under drearburh’s shadow. they can trust each other. then harrow, with her seventeen years of pining, obsession, and isolation, asks gideon something truly fucking awful:

"Harrow said, "In the event of my death--Gideon, if something ever does get the better of me--I need you to outlast me. I need you to go back to the Ninth house and protect the Locked Tomb. If I die, I need your duty not to die with me."ALT

gideon the ninth, chapter 35

in harrow’s mind, the locked tomb (specifically) has served as her reprieve. it is The Thing For Which She Suffers It All, and so it must be good. harrow sees herself as the thing that poisoned gideon, because she IS the ninth house, and she hates herself. I genuinely don’t think harrow understands the depth of cruelty in this ask at all. in harrow’s mind she is saving gideon and protecting The Body.

and as cytherea bears down on them, harrow reminds her:

""Griddle, you made me a promise. You agreed to go back to the Ninth. you agreed to do your duty by the Locked Tomb--" "Don't do this to me." "I owe you your life," said Harrowhark, "I owe you everything.""ALT

gideon the ninth, chapter 37

if harrow sacrifices herself, gideon owes her AND gets abandoned. gideon has to go back and protect the tomb, back to a life she could not bear long before she had ever seen the sea and the sky, before kind-hearted house scions and princesses with swords. the chains would be slapped on. gideon would not get out twice. not even in a box.

gideon can die neatly and heroically, like a protagonist in a comic book—she can save harrow, save camilla, get vengeance for lost friends, carry out the last wish of jeanmarry and isaac, who she so badly let down—or she can live and return to drearburh.

"Harrow said-- "I cannot do this." "You already did it," said Gideon. "It's done. You ate me and rebuilt me. We can't go home again."ALT

gideon the ninth, chapter 37

"Gideon lay smiling a small, tight, ready smile, stretched out beneath a blue and foreign sky."ALT

gideon the ninth, chapter 37 (final line before the epilogue in harrow’s POV).

of course she chooses the fence.