I think yukyuyama is right – gerade is added to indicate that you’re doing something at that moment (rather than in general).
Just for information though, I think there are some dialects, for example Westphalian, which have an equivalent to the ‘continuous’;
Ich bin am lesen = I am reading.
But, bear in mind it’s not Hochdeutsch.
How charmed I am when I overhear a German word which I understand! - Mark Twain