gotta = got to. Its a casual way or a slang to say it.

Avoid this slang expression in formal writing or formal conversations as it shows that you are a poorly educated person.

Hey whitney I suppose Bill has given you the right answer....

"I gotta do it" is a highly contracted way of saying, "I have got to do it", which is a grammatically awkward way of saying, "I have to do it". Yeah, I tend not to talk that way, personally.

since you need to learn formal English for your success, I am not advising you to worry about gotta or wanna It's just for the the informal speech. it's American dialect or street English. if you start to speak that kind of English, you can keep talking it even when you are on an Interview for a job, and you can miss the job, so do not pay attention to that.