[2] After graduation he joined the American Methodist Episcopal Mission and was assigned to Beijing, China – called Peking at that time – as a missionary and the principal of a boys' school.

Over the next several years he supervised construction of churches and other buildings on the Methodist compound, the largest of the Protestant missionary compounds in Beijing[5] In early 1900 an anti-foreign, anti-Christian peasant movement spread northward from Shandong Province taking over control of much of the countryside, burning churches and killing Chinese Christians.

The Boxers, as the participants in the movement were called, had substantial support within the Qing dynasty government and from the Empress Dowager Cixi in Beijing.

The Gamewells were planning to leave Beijing by train on June 5, 1900, en route to a furlough in the United States.

The Church was converted into a fortress..."the altar was fenced around with a barricade of boxes of condensed milk, biscuit tins, baskets of household silver, etc.

While the missionaries were fortifying their compound the Boxers were raging through Peking destroying foreign establishments and executing Chinese Christians.

Foreign soldiers in the Legation Quarter exacerbated the situation by firing on Chinese demonstrators and mobs and killing many people.

[9] On June 19, the already ominous situation in Beijing took a turn for the worse when the Chinese government ordered all foreigners to leave the city within 24 hours.

He gave Gamewell absolute authority to organize the fortification of the British legation against an anticipated attack by Boxers and the Chinese Army.

Already the British Legation, which at the commencement of the siege was utterly undefended by any entrenchments or sandbags, is rapidly being hustled into order by the masterful hand of this missionary ... the hard worked man always finds time for everything.

"[12] Gamewell wanted thousands of sandbags and American missionary women scoured the Legation Quarter for sewing machines and cloth.

Curtains, silks, satins, damasks, and expensive cloth of every kind was cut and sewed by missionary women into bags, filled with dirt, and placed on Gamewell's barricades.

At one strong point he had a barricade built eight feet thick, consisting of brick and rubble and earth and capable of withstanding cannon fire.

Gamewell set about digging counter mine trenches ten to twelve feet deep to surround the British legation.

Gamewell was still strengthening his fortifications when an allied expeditionary force raised the siege and rescued the foreigners and Chinese Christians within the Legation Quarter on August 14, 1900.