Thanks a million S M Blinder...
In[14]:= GCD[Mod[4, 5], Mod[4, 11], Mod[4, 13]] Out[14]= 4
Since you chose WolframAlpha as a category for your question, I assumed you wanted an answer in terms of WolframAlpha
Click here to go to the WolframAlpha answer
You may find it easier to get correct answers if you do not try to "desktop publish" the questions you are giving to WolframAlpha