Personally, I use Krita (which is also an open source software) for creating GIFs.
My steps :
- In Blender, Ctrl+F12 to Render an Animation (be sure that your Output Properties are PNG Images),
- In the folder, choose the first image an open it in Krita
- Ctrl+A to select all and Suppr. to delete the image
- In File, click on Import animation frames
- Select your images from number one to the last one. Be careful of the naming. It will import them into your open image as sort of hidden layers.
- Go to File again, click on Render Animation
- First frame is 0, it means that your first image is the zero. So in Last frame, type the number of your last image minus one.
- Select the number of FPS your want
- And thats it.
Hope my English is readable enough. Enjoy !