Gao's Grand Mosquée is a local religious landmark rather than a showpiece monument, but it belongs in the same mental map as the market, river crossing, and Tomb of Askia. The city was one of the great centres of Songhai power, and its mosques are reminders that Gao's importance was commercial, political, and Islamic all at once.
Non-Muslim visitors should not assume they can enter. The value here is mostly in seeing how the mosque sits in the living city, especially if you are walking with someone local who can explain the neighbourhood and the rhythm of prayer, market, and river life.