I'm not really sure what your full code looks like or what tool you seem to be using but because of how this function is designed to work the return value contains multiple bits of data and thus is an object (and must be treated as such).
I downloaded the gaussian code from GitHub and took a look to see what was in the object. There are 3 values stored: mean, standardDeviation and variance. Obviously the mean and variance were plugged into the function so I assume you want the value of standardDeviation.
Again I don't know how your tool works or what your actual code looks like but I guess you do have 2 ways of getting the value you want.
// Store the object and then call the value
var distribution = gaussian(mean, variance);
// Directly call the value from the function
I know you say you can't use alert() or console.log() but alert() was used in this example to simply show the values being printed out somewhere. Hopefully you do see how you could use the code (minus the alert() function) to adapt what you have and get your value directly.