The answer was in the same discussion. The conversation was:
Gamora: I was a child when you took me.
Thanos: I saved you.
Gamora: No, no, we were happy on my home planet.
Thanos: Going to bed hungry, scrounging for scraps? Your planet was on the brink of collapse. I was the one who stopped that. You know
what's happened since then? The children born have known nothing but
full bellies and clear skies. It's a paradise.
Gamora: Because you murdered half the planet!
Thanos: A small price to pay for salvation.
Gamora: You're insane. - IMDb
He saved her from her planet's collapse. According to Thanos her planet was on the brink of collapse. And he saved her from that future by killing half of the life on that planet. That planet is now a paradise because of him.