The onset and physical patterns of gap instability (GI), characterised previously as an inviscid, Kelvin–Helmholtz instability, were investigated experimentally and numerically in an eccentric annular channel with an inner-to-outer diameter ratio $d/D=0.5$ and a length equal to 320 hydraulic diameters. The focus was on the range of low and moderate eccentricities ($0\lesssim e \leqslant 0.5$) and Reynolds numbers ($\textit {Re}\lesssim 12\,000$). It was found that, in laminar flow, GI occurred for $e$ as low as $0.05$. When, however, the flow was turbulent in at least part of the cross-section ($\textit {Re}\gtrsim 5000$), GI remained strong only for $e\geqslant 0.5$ but was essentially undetectable for $e\lesssim 0.3$. For $e$ lower than 0.3, the critical Reynolds number for the onset of GI increased with decreasing $e$. Time-frequency analysis of the velocity time histories has revealed the presence of a single type of dominant mode for $e=0.7$ and all considered $\textit {Re}$, and two distinct, occasionally coexisting, modes for $e \lesssim 0.5$ and $2000 \lesssim \textit {Re} \lesssim 5000$. Within a range of low-$e$ or low-$\textit {Re}$ conditions, quasi-periodic flows were highly intermittent and less energetic in an upstream section of the channel, but became progressively less intermittent further downstream. The energy of such motions generally increased with increasing streamwise distance, e and Re. By exception, this energy decreased with increasing $\textit {Re}$ for $e\leqslant 0.3$ and $\textit {Re}\gtrsim 2000$. The gap vortex street generation mechanism and development was analysed and an improved physical model was proposed.