There is quite a pile of literature on the subject of the inverse of a Gaussian distributed variable (this should not be fixed with inverse Gaussian distribution – it is a different matter). In fact, the inverse distribution is ill-behaved; the mean and variance does not generally exist.

I came up with a simple approximation that works well if the mean is large enough and the variance is small enough (I have not worked out the details of the exact conditions for this approximation. However, the results can be verified by Monte Carlo simulations).

First, approximate the Gaussian distributed variable by a log-normally distributed variable Lognormal with corresponding mean and variance, i.e.

Using the theory of log-normal distribution, the inverse of is now given by

Lognormal

That's it!

References:

- Log-normal distribution
- Díaz-Francés, Eloísa; Rubio, Francisco J. (2012-01-24). "On the existence of a normal approximation to the distribution of the ratio of two independent normal random variables". Statistical Papers. Springer Science and Business Media LLC.