Maybe this is rather a comment then an answer but two points:

 - GBM is a stochastic model for stock prices. It is used to price derivatives in an arbitrage free setting. In this case you look at a process whose expected return is just the risk free rate (due to no-arbitrage). Forecast this price is trivial.
 - It is debatable how forecastable stock prices are. If you want to learn about forecasting in a statistical sense then you could look at Rob Hyndman's resources (papers, R packages, free [online book](https://www.otexts.org/fpp/)).