The Gambia's new government has asked the United Nations to halt the process of withdrawal from the International Criminal Court (ICC) begun by the regime of former leader Yahya Jammeh.
The Gambia is to rejoin The Commonwealth within months - after four years outside the group of 52 states.
The visit is coming weeks after Gambian President Adama Barrow exclusively told IBTimes UK he wanted to strengthen relations between Gambia and the UK by, among other things, rejoining the Commonwealth and promoting foreign investments.
The incident happened during Friday prayers at the King Fahd mosque in Banjul, the military force sent by the Economic Community of West African States (ECOWAS) said in a statement.
Mr. Barrow, who was sworn in last month, has also said The Gambia will reverse its move to withdraw from the International Criminal Court (ICC).
Last week, the European Union promised The Gambia an aid package of nearly £65m ($81m) - almost three years after freezing its assistance to the West African nation.
Mr. Jammeh, who ruled Gambia for more than two decades, initially refused to accept his election defeat, but after pressure from Senegal fled into exile in Equatorial Guinea.
Also during his trip, Mr. Johnson will meet President Nana Akufo-Addo of Ghana. "Their elections highlight the continuing strengthening of democracy in West Africa", Johnson said ahead of the trip.