You can say "I am done with my homework" or "I finished my homework" to say that your homework was completed. Or, you could say "I stopped doing my homework" if you were working on it, but did not finish it.
Saying "I am gone" is saying that you have already left the place that you were at. You would probably say "I'm already gone" or "I already left" or just "I left"
Example: Someone calls you on the phone and asks if you are at a party. You say, "I already left" if you are no longer there anymore.
'Leave' has several definitions, and you can say it different ways
You would say "I will leave" if you haven't left yet, but you will in the future
Example: 'I will leave after I finish eating"
If you are in the process going from a place, you would say "I am leaving"
Example: You would say "I'm leaving!" to you mom as you walk out the door
Hope this helped!