Hi,
There is no iMovie feature that directly addresses your issue.
As I understand it, you have a gif of 0.56 seconds duration that you want to replicate as many times as necessary to create a longer duration gif to mix into your movie. Is that correct?
If correct, there is no way to avoid some copy-pasting. Create a new project for the gif. Import the gif into it. Edit/Copy and Edit/Paste it into the timeline. Do an Edit/Paste several more times. You now have a string of gifs in the timeline. Select the whole range of gifs as a batch and copy-paste them repeatedly into the timeline until you reach the duration that you want. Then share out the project and import the shared out movie into your original project as one long clip of replicated gifs. Does that do what you want?
-- Rich