Hi ! I think someone started working on it but they stopped pretty early in the implementation.
The current code is here 2 and it appears that the _compute method is incomplete/not working. This method is supposed to return a dictionary of metrics (for example {"gleu_mean": mean, "gleu_std": std, "gleu_ci": ci}) from the list of references and predictions.
If you’re interested in contributing, feel free to try to make this method work and create a PR 
Locally you can test your metric with
from datasets import load_metric

gleu = load_metric("path/to/gleu.py")
gleu.compute(refererences=references, prefictions=predictions)