GRU4Rec is the original Theano implementation of the algorithm in "Session-based Recommendations with Recurrent Neural Networks" paper, published at ICLR 2016 and its follow-up "Recurrent Neural Networks with Top-k Gains for Session-based Recommendations". The code is optimized for execution on the GPU.
At this point if a developer uses the 'item_key' argument with a value other than 'ItemId', it can not create/evaluate a model correctly.
This pull request solves the problem.