(GRU, in case anyone is wondering, stands for "gated recurrent unit" and is a building block of standard LSTMs)
EDIT: now that I've finished the paper, I've realized that the citations are straight-up missing. That's no good, but I'm sure the authors just messed up the arxiv upload. If OP knows them, they should let them know... failing to include any citations at all is a quick way to decrease the credibility of an article.
We are trying to fix the Latex problem and submit a replacement to ArXiv soon. In the meantime, we hosted the PDF version of the paper on another link:
Thanks and look forward to your feedbacks and suggestions :)