Honestly, this reads like a rough translation. This isn't writing by a native English speaker. From this paragraph, it's not clear what exactly they mean by 'shorter walk'. It suggests that they were going to take a different route to the other person's house, which is a 'shorter route' (it's closer).
We would only say "walk in the fresh air", not "walk in gravity"
Here's the wikipedia link for gravity. http://en.wikipedia.org/wiki/Gravity
We would only use it in science or maybe to make a joke.
For example, if you drop your pencil, you'd joke, "darn gravity!"