Does Pi change when gravity changes
$\pi$ is just a number. It doesn't vary with anything.

The ratio of the circumference to the diameter of a circle varies with geometry, so in the general case, this definition doesn't hold. But there are other definitions, such as
$$
\pi = \int_{-1}^{1}\frac{dx}{\sqrt{1-x^2}},
$$
or
$$
\mathrm{The\,smallest\,positive\,number} \, x \, \mathrm{for\,which}\, \arccos(2x) = 0.
$$
The latter definition can also be expressed a power series, which is independent of geometry.