[TeeWEE]

An "Electro chemical gradient" is an ion. That works on small scale within cells. The gradient here is the "electrochemical potential" of the ion.

An Synthetic Gradient is a way to allow learning Forward Propagated Neural Nets in a parallel way. The gradient here is referring to the 'error' backpropagation that is part of the training process of an neural net (im talking about computer science neural nets).

They have nothing todo with each other. The papers that you are referring to have nothing todo with the process of training a neural net.