Why is it not "Dw i'n gobeithio" or "gobeithia"?
gobeithio... on its own is commonly translated as 'hopefully...' To distinguish between 'I hope...', he hopes...', and so on it is more usual to see dw i'n gobeithio..., mae e'n gobeithio..., ...
so are you saying that this translation isn't right?
Yes. The answer has now been corrected to 'hopefully' - it may take a short while for the change to filter through the system.
Can it translate to "Hoping that you are ready."? In a letter, we can put, "Hoping that you are well," which is the shortening of "I'm hoping that you are well." Is there some nuance to the Welsh expression?
'Hoping that you are ready.' would be grammatically correct English, but just an odd thing to say as a sentence on its own.