How to edit gif animation?

Question

gif

Hello!

The customer wanted animation for his site, since I had never worked with animations before, I went to the Internet for ready-made solutions. I found a suitable option, but the right side of the door needs to be slightly reduced:


With what program and how can this be done?
If you draw animation yourself, what is the input threshold here? Is it possible to learn how to make such simple animations in an evening?