I don't really like to give non-answer answers, but if you want to compute indefinite integrals, I would suggest using something like WolframAlpha, or Wolfram Mathematica as opposed to Matlab.
If you actually just want to compute a definite integral, then you can do this.
integral(@(s) s .* f_gamma(s, n), 0 , W)
Before, you were trying to muliply a number with a function, which mathematically is fine, but programmically isn't since a function is pretty complicated. This code snippet will create a new function whose content is s .* f_gamma(s, n), then integrate it. Of course, I hope the variable n is already defined yes?
Read more here https://www.mathworks.com/help/matlab/matlab_prog/anonymous-functions.html