The question is not completely clear to me.
Either you are asking what is min/max gravity for unmodified Humans in any particular game, which can be found in Race details/Races window as Maximum Deviation(G) and can be set when creating new game, if I remember correctly. In current version default is 0.9G, which is also max.
Or you are asking what is it for actual humans? In that case the answer is we don't know. Simply we haven't tried so we literally don't know. As far as I know there have been some experiments with things like flies and mice, but hey were in microgravity, not reduced/heightened gravity - say by tenths of G, or at least way less of them. What I'm talking about is of course prolonged exposure. Short term effects we know much more about, but those are of course of little use when talking about colonizing. We know micro is problem, but what about Moon or Mars gravity? First, can humans survive it long term? Years, decades? And more importantly can they have children? What is known is that some development is time critical, you have to have needed stimulus at the time or things don't develop right. You could put expecting mothers in what is basically huge centrifuge which goes to countering low g, but it gets real expensive real fast if you have to keep this up for months or years. As for high g you can have apartments real high on some structure like space elevator. Returning back to long term survival it may vary significantly for individuals, say some part of population might suffer from strokes under higher gravity. This also applies, to lesser degree to atmospheric pressure and oxygen content. We have on earth places with differing oxygen content and atm pressure, sea level to high mountains, where people develop normally. Anything out of that is untraveled road, considering long term. Also different gas mixes. And thermal conductivity of said gas mixture. That could move temperature tolerances.
Also note that the game is less than perfect. It allows me to colonize Io at the same cost as other Galilean moons.