Nanoindentation experiments are performed at the vicinity of grain boundaries, in Fe-Si tricrystals, to illustrate the existence of a critical stress at which slip transmission occurs across grain boundaries. Such a critical stress can be considered as a grain boundary yield stress and can be quantified within the framework of conventional gradient plasticity theory, enhanced by introducing a new mechanically induced "interface energy'' term. The present study takes a first step in trying to provide a physical interpretation for this "far from thermodynamic equilibrium'' interface energy term by conducting nanoindentation tests in three Fe-3wt%Si tricrystals, each of which had three distinct types of grain boundary misorientations, namely 22.5 degrees, 42.0 degrees and 44.6 degrees. By relating the experimentally measured grain boundary yield stress to the predictions of interfacial gradient plasticity, it is possible to determine the interface parameter (n), which provides a measure of the resistance to slip transmission for each grain boundary examined. In particular, microscopic arguments from standard dislocation theory reveal that n depends on both the grain interior properties and the grain boundary structure. The internal length is shown to depend on multiple characteristic lengths of the microstructure, while a new expression is deduced for relating the Hall-Petch slope to both the interface parameter and internal length.