What is the algorithm used by fit function for Gaussian fit?

I am working on a project and it involves fitting data to multiple Gaussians. I need to establish/find existing theoretical bounds on the errors on the estimated parameters. Hence I need to know the algorithm that is used in fit function and how are the confidence intervals found? I did google up for standard nonlinear squares but couldn't find the methods to find the confidence intervals. Any help would be appreciated. Thanks!