In an etf a persistent gap usually means the NAV is stale rather than the price wrong: an international fund's holdings stopped trading hours ago, so the ETF price is the live estimate and the NAV is history. In a closed-end-fund there is no arbitrage at all, so the gap is a genuine sentiment premium or discount.
Watch premium spikes as a stress signal. An etn whose issuance is capped, or an ETF whose creation-redemption is impaired, can detach from fair value by amounts that dwarf any view you had on the underlying.
Example: ETF price $41.55, NAV $41.00. The premium is $0.55 / $41.00 = 1.34%. Buying there means paying 1.34% above the assets, and the gap typically closes when the underlying market reopens.
Related: nav, creation-redemption, closed-end-fund, etn, authorized-participant