You start by writing the formula for it. That can be found on virtually the first line in this link.
However in MATLAB, the gamma function seems to be defined only for a real variable, unless you use the symbolic toolbox version of gamma. So this should work:
t = (1:5)';
[t,angle(double(gamma(sym(1/4 + i*t/2)))) - log(pi)/2*t]
ans =
1 -1.76754795281229
2 -2.52591091881613
3 -2.99456469601083
4 -3.29063503121648
5 -3.45962037536346
That is valid for real arguments t. I'd need to do some reading to know if this works for complex arguments, though I assume it does not, since the wiki link I gave indicates it is valid for real arguments. Hopefully you care only about real arguments, and are uninterested in an extension to the complex plane.
