原句应为sticks,木棍,树枝
gather somebody something 给某人搜集某物 相当于 gather something for somebody
也就是说gather him all the dry sticks=gather all the dry sticks for him
最后的they could find是用来描述sticks,可以理解为all the dry sticks that they could find,只不过这里面的that被省略了.
翻译：他们把能找到的干树枝都搜集给他了.