The statement uses the word "gotta" which is slang for "got to" or "got to be". The non-slang version of that statement is: "You've got to be kidding me". You'll notice that in the non-slang version, the infinitive of "be" is used which is "to be". Here's a link to what this English idiom means: http://idioms.thefreedictionary.com/You%27ve+got+to+be+kidding! I hope this helped! If not, just let me know, and I will explain further. -Kyle

It's a sentence used when you are not believing something someone said or when someone says something strange Be is used because after gotta or "got to" you should use infinitive of the verb