While Granger Causality (GC) has been employed in a large number of problems in network neuroscience, the vast majority of GC applications are based on linear multivariate autoregressive (MVAR) models. However, it is well known that real-life systems (and biological networks in particular) exhibit notable nonlinear behavior, hence undermining that validity of MVAR-based GC (MVAR-GC) approaches. Current nonlinear approaches to GC estimation only cater for additive nonlinearities or, alternatively, are based on recurrent neural networks (RNN) or Long short-term memory (LSTM) networks, which present considerable training difficulties and/or the need to be carefully tailored to specific applications. We define a novel approach to estimating nonlinear, directed within-network interactions based on a specific class of RNNs termed echo-state networks (ESN), where training is replaced by random initialization and the internal basis is built through orthonormal matrices. We reformulate the classical GC framework in terms of ESN-based models for arbitrarily complex networks, and characterize the ability of our ESN-based Granger Causality (ES-GC) estimator to capture nonlinear causal relations in a network of noisy Duffing oscillators, showing a net advantage of ES-GC in detecting nonlinear, causal links. We then explore the structure of ES-GC networks in the human brain employing functional MRI data from 1003 healthy subjects scanned at rest at 3T within the human connectome project, demonstrating the existence of previously unknown directed within-brain interactions. In summary, ES-GC performs better than commonly used and recently developed GC detection approaches, making it a valuable tool for the analysis of e.g. multivariate biological networks.