"I have to go" and "I have got to go" are almost identical; "I have got to go" is a little more casual than "I have to go". "I gotta go" is the most casual way to say those two sentences but most of the time, it is not used literally. 我要走啦 = I have to go/I have got to go. 我走啦 = I gotta go Hope it helped!