factors.groovy

def factors(int n) {
    (2..<n).findAll { i -> n % i == 0 }
}

// FIXME. If you provide, e.g., 15 and 30 it should return [3, 5, 15]
def commonFactors(int... nums) {
    def commonFactors = factors(nums[0])
    for (int i = 1; i < nums.length; ++i) {
        commonFactors = commonFactors.intersect(factors(nums[i])) 
    }
    commonFactors
}

def gcf(int... nums) {
    commonFactors(nums).max()
}