A hermit crab retracts into its shell when a shadow passes over it
A flower closes when darkness falls
A slater moves away from a light source
A fish moves towards a particular smell/chemical
Because you lie down to sleep so you are not always upright. If it was a response to gravity you would always be upright.
Because gravity is a fairly constant force on Earth so there is no change to respond to. And you don't stand up straighter if you go up a large mountain for example.
The being 'right side up' is not a response of your whole body towards the 'stimulus' of gravity. If you were put in the ocean and then swam towards the bottom automatically, that could be a positive gravitaxis. (Not a very advantageous behaviour for your survival though)
Gravitaxis is limited to small or unicellular organsisms. We are large organisms and gravity acts passively on us, No part of us at any stage in our life cycle responds to gravity.