Question

What does Bryan mean when he says there is “no Gatling gun attachment” to preaching the Bible ?

Answer 1

Some argue that American rule in the Philippine Islands will result in the better education of the Filipinos. Be not deceived .... (We) dare not educate them lest they learn to read the Declaration of Independence and the Constitution of the United States and mock us for our inconsistency . 
... (A) war of conquest is as unwise as it is unrighteous .... It is not necessary to own people in order to trade with them .... 
Imperialism finds no warrant in the Bible. The command, "Go ye into all the world and preach the gospel to every creature," has no Gatling gun 
attachment. .. 

Answer 2

Answer:

He meant preaching of gospel did not have any relationship with imposition  of  religion via violence, speccially speaking to the use of guns.

Explanation:

William Jennings Bryan  was against to the spread of ideologies by imposition, by submission of other cultures. He was against impreliasim and rejected the idea of using any kind of violence to preach the gospel. The Gatling gun was a very known weapon on his time and this why he used the phrase "no Gatling gun attachment" as reference to his vision of preacing  without guns, swords nor any violent means.