If you want to use English properly - and you want people to take you seriously - you should not write either of these forms. They look careless and lazy, and give a bad impression to the reader. The correct forms are: 1. ' I have to go' or ' I've got to go' This is similar to 'I must go' or 'It is necessary to go' 2. 'I am going to go' or 'I'm going to go' This is similar to 'I'll go'. It means that it is my plan or intention to go. I hope this is clear.

gotta = got to gonna = going to "I going to go" is wrong. It should be "I am going to go".