So, looking firat at brother William John. If he died as recently as 2003 in Bath then you can be fairly sure that the death entry in the indexes for Dec 2003 is the right guy, and that gives a d o b as 26 April 1912.
The birth indexes for that person show he was born in Taunton, mother's maiden name Moss. And you say you have that cert, giving parents as Ernest Gale and ALice nee Moss. So if you are sure that the guy who died only a few years ago in Bath was the right one, then you can be sure that this is the right birth.
Now, you say you have a birth cert for your grandad Edward Walter, also giving the parents as Ernest Gale and ALice nee Moss. So you can be sure that this is the right cert for your grandad, and he was actually born in Cardiff and not Hereford as family rumour had it.
So that is your original question answered - you have his birth date and location!
To go further back is harder, like you I have found the marriage of an Ernest Gale to a Lily Moss on FreeBMD.
You may be best trying to find Edward Gale on 1901 census next, difficult as you have no approx d o b for him.
Or alternativley trying to find his death - you know it is after 1912 when William born ,but before whatever date son Ernest Walter marries in the 30's.
other folk may have better ideas