gCF :: Integer -> Integer -> Integer;
gCF n p
    | (p <= 0 || n <= 0) = error "Input should be positive"
    | (p > n) = f p n
    | otherwise = f n p
    where
        f :: Integer -> Integer -> Integer;
        f a b
            | (fromInteger (a `div` b) / 1 == a / b) = b
            | otherwise = f a (b - 1)

testscript.hs:168:28: error:
• No instance for (Fractional Integer) arising from a use of ‘/’
• In the first argument of ‘(==)’, namely
    ‘fromInteger (a `div` b) / 1’
  In the expression: (fromInteger (a `div` b) / 1 == a / b)
  In a stmt of a pattern guard for
                 an equation for ‘f’:
    (fromInteger (a `div` b) / 1 == a / b)
    |
168 |                         | (fromInteger (a `div` b) / 1 == a / b) =      b     |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^