GRU and LSTM are two popular RNN variants out of many possible similar architectures motivated by similar theoretical ideas of having a "pass through" channel where gradients do not degrade as much, and a system of sigmoid-based control gates to manage signals passing between time steps.
Even with LSTM, there are variations which may or may not get used, such as adding "peephole" connections between previous cell state and the gates.
LSTM and GRU are the two architectures explored so far that do well across a wide range of problems, as verified by experiment. I suspect, but cannot show conclusively, that there is no strong theory that explains this rough equivalence. Instead we are left with more intuition-based theories or conjectures:
GRU has less parameters per "cell", allowing it in theory to generalise better from less examples, at the cost of less flexibility.
LSTM has a more sophisticated memory in the form of separating internal cell state from cell output, allowing it to output features useful for a task without needing to memorise those features. This comes at the cost of needing to learn extra gates which help map between state and features.
When considering performance of these architectures in general, you have to allow that some problems will make use of these strengths better, or it may be a wash. For instance, in a problem where forwarding the layer output between time steps is already a good state representation and feature representation, then there is little need for the additional internal state of the LSTM.
In effect the choice between LSTM and GRU is yet another hyperparameter to consider when searching for a good solution, and like most other hyperparameters, there is no strong theory to guide an a priori selection.