When to use GRU over LSTM?
GRU is related to LSTM as both are utilizing different way if gating information to prevent vanishing gradient problem. Here are some pin-points about GRU vs LSTM-


The GRU controls the flow of information like the LSTM unit, but without having to use a memory unit. It just exposes the full hidden content without any control.
GRU is relatively new,  and from my perspective, the performance is on par with LSTM,  but computationally more efficient (less complex structure as pointed out).  So we are seeing it being used more and more.


For a detailed description, you can explore this Research Paper - Arxiv.org. The paper explains all this brilliantly.

Plus, you can also explore these blogs for a better idea-


WildML  
Colah - Github


Hope it helps!