How does type inference works on this?

MinskyGADT.hs

module MinskyGADT where

import           Data.Array
import qualified Data.ByteString as BS
import           Data.Word (Word8)

data GArrayLike a where
  GArray :: Array Int a   -> GArrayLike a
  GBytes :: BS.ByteString -> GArrayLike Word8

gLength (GArray a) = snd (bounds a)
gLength (GBytes b) = BS.length b

-- :t gLength
-- gLength :: GArrayLike e -> Int