“Gotta go now. Bye!” This is a common casual way to end a phone conversation. But it’s good to remember that it’s a slangy abbreviation of the more formal “I have got to go now.” In writing, at least, remember the “have” before the “got” in this phrase meaning “have to.” In fact, you can omit the “got” altogether and say simply “I have to go.” For a slightly less formal effect, contract “have” thus: “I’ve got to go.”