Firstly, 'so' isn't correct. 'Really' is better in this case. 'It's got to be really hard' doesn't make sense.. I'm not sure what you mean - "It's bound to be really hard', maybe? (of course it will be very hard - those exams are always hard) It's going to be really hard = a future sentence 'gonna' is like slang for 'going to' :)

Expanded for explanation It's got to be so hard - "It has to be so hard" This is present tense, "it" must be "so hard" It's gonna be so hard - "It's going to be so hard" This is near future tense, "it" will become "so hard" at some point in the future