Implementing Minimum distance estimation Let $\mu$ and $\sigma$ be two parameters of interest characterising a normal distribution. From a theoretical model, I know that these two parameters are related to each-other according to 
$$\pi=\Phi\left(\frac{\Phi^{-1} (\alpha)-\mu}{\sigma}\right)$$
where $\Phi(\cdot)$ is the CDF of the standard normal and $\pi$ and $\alpha$ are random variables, for which I observe a number of realisations (let me note the empirical realisations $\hat{\pi}_i$ and  $\hat{\alpha}_i$ for $i=1,2,..n$).
Based on this, I would like to estimate $\mu$ and $\sigma$ using minimum distance, namely
$$
\arg\min\limits_{\mu,\sigma}\sum^n_{i=1}\left( \hat{\pi}_i- \Phi\left(\frac{\Phi^{-1} (\hat{\alpha}_i)-\mu}{\sigma}\right)\right)^2 w_i
$$
for some vector of weights $\{w_1,w_2,..w_n\}$.
How do I implement that in STATA? If my understanding of the methods is correct, I believe I should be able to do this using the General Method of Moments. I tried using gmm in Stata, but it gave me an error message. Specifically, I tried
gmm (pi - normal((invnormal(alpha)-{mu})/{sigma})), instruments(pi alpha)

but received an error message saying 
no non-missing values returned for equation 1 at initial values
 A: You need to change the initial value of sigma. Default is zero, so you're dividing by zero and get missings at your initial values, as your error message indicates.
A: The gmm code provided in the OP is correct, but it requires an initial condition for $\sigma$. Here is a working example.
clear
set obs 10000
gen alpha       = rnormal()
gen alpha_cum   = normal(alpha)
scalar mu       = 2
scalar sigma    = 3
gen pi_real     = normal((invnormal(alpha_cum)-mu)/sigma)
gen error       = rnormal(0,1) 
gen pi_observed = pi_real + error
gen z           = rnormal()

Running  gmm (pi_observed - normal((invnormal(alpha_cum)-{mu})/{sigma})), instruments(alpha_cum pi_observed) gives the error message: no non-missing values returned for equation 1 at initial values
Running  gmm (pi_observed - normal((invnormal(alpha_cum)-{mu})/{sigma=1})), instruments(alpha_cum pi_observed) 
Gives 
------------------------------------------------------------------------------
             |               Robust
             |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
         /mu |   1.737236   .1612666    10.77   0.000      1.42116    2.053313
      /sigma |   2.702077   .2730999     9.89   0.000     2.166811    3.237343
------------------------------------------------------------------------------

