in module/geb-core/src/main/groovy/geb/navigator/DefaultNavigator.groovy [1022:1035]
protected boolean setCheckboxValue(WebElement input, value) {
if (value == null || value == false || (value instanceof Collection && value.empty)) {
unselect(input)
} else if (value == true) {
select(input)
} else {
def values = value instanceof Collection ? value*.toString() : [value]
if (getValue(input) in values || labelFor(input) in values) {
select(input)
} else {
unselect(input)
}
}
}