Well, in comic-verse, Gamora was a cluster of retcons and character-deaths that would get retconned through 'magic'.
-Story-wise, she had already died, like, four times. Including being totally erased from ever existing at least once. [I]She got better.[/I]

Trying to shove that shit and drama (time travel, warlock, two reincarnations*, and a series of pre-pre-emptive genocidal rampages) into a single movie got trimmed down to "She's a time-paradox; Thanos did it".
*at one point she got sucked into a rock for a few years.

Not sure what was up and off with Drax, though. Maybe they couldn't fit enough of Gamora's backstory on Gamora so they invented Drax (which is partially true, see also the Phalynx hive-mind).