The correct writing of the first word(s) is 'got to' - 'I've got to' and 'I must' shows some form of obligation/necessity to do the action which follows. 'Must' is more formal. Hope this helps Bob

'Must' a modal verb indicating obligation. 'Gotta' isn't a word at all - it's a sound you hear when someone says 'I've got to' in a relaxed and fluid way.

Gotta is informal and not usually used in writing. Must is is fomal. They both mean the same thing: have to . Eg, I have to leave now, I gotta leave now, I must leave now.

I think they give the same meaning..However, must is formal and gotta is informal