Agree with the above statement that this is considered blunt and not polite. Here is a way that you can head out on your way, with a smile and a friendly look in your eye. "It's been so nice talking with you, but I've got to get on my way." This way you have given them 3 positive messages before the negative one. 1. A smile. 2. A direct look into their eyes. 3. A compliment on your enjoyment of being in a conversation with them. If you and the receiver are comfortable with physical communication a 4th can be used. 4. A handshake, a touch on the arm, or even a hug goodbye. Eveyone is busy. When you say you've got to go, using these positive reinforcements, the other person feels good enough about the conversation that their feelings are spared. If you simply want to move on to speaking to someone else and are not leaving, it is acceptable to simply say, "It's been so nice talking with you!" as you get up with a smile, a friendly look, and and physical touch (if desired). Grammar and culture. :D

Both are perfectly fine, but with slightly different meanings. "I'm leaving now" is a statement about what you are doing now: it is already happening, so it gives a real sense of urgency; you are simply informing somebody about what you are already doing. Your other three options all use the future tense, but refer to 'now'; this slight contradiction gives the sense that you have just decided to leave: you are describing an intention or plan, which you intend to act on immediately.

If it’s urgent that you go now or you have told someone that you must go, you can say “I must go now” or “I have to go now”. If you are casual and just decide to go, just say “I’m gonna go now” (very informal) or you can still say you “have to go”. The phrases in numbers 1, 2 and 4 sounds like responses to being asked to leave and you really wouldn’t use those in a casual comment before leaving. Americans are not that polite sometimes but if you preface your comment with “I’m sorry but..” then “I have to go now”, it would be most American!

Often, in English, being less direct with your language is considered more polite. 1. I'm leaving now. 2. I'll leave now. 3. I'm gonna go. 4. I'll go now. - All of these are true, you are going to go. You have decided! But the way we give people this information can change how they feel about it. "I've got to go" sounds like, "I would like to stay and continue this conversation with you, because you are lovely and interesting, but, I have other things that I must do. So I have no choice but to leave". Which is a really nice thing to say! But a bit too long...