Hi Denise,
I'm not sure what you mean by "When Charles Gordon's mother Catherine Gordon Nee Mason dies in 1921 William is not mentioned in her death notice or memorial notices"
Charles may have kept William and accepted him as his own (as per Catherine's memorials notices impress) but that doesn't mean Charles family was happy or accepting of William. (as Charles death cert implies)
It could have been considered improper and disrespectful by Charles family to mention William in their mothers death/memorial notices. By 1921 William was a grown man and could have placed his own memorial notices if he felt attached to Catherine d.1921.
Kellie