Gringo
The story I've always heard was the US soldiers wore drab green uniforms and the local population, not overly pleased with their presence said "green go".
Over time the green go sounded more like gringo.
If you study Dominican history you will find the US had a military presence in the country several times the past century. Mainly as security to the government administrators dispatched to oversee and control the Dominican customs operations. They had been taken over by the US as tremendous debt was owed not only to the US but several European countries, the latter of which were threatening obtaining payment by force. A hallmark of Dominican history seems to be financial mismanagement.
Much of the intrastructure of the country was put in place by US works projects in the late teens and 20's, including roads, bridges and sanitary systems.