Ever since that thought entered Granny Shang's mind, she had been wondering about how to get Rong Mo and Shang Mo together.
She definitely couldn't go about it just by telling Shang Mo that Rong Mo was a girl.
Firstly, she had promised Rong Mo to not reveal her gender to any outsiders. A promise was a promise.
Also, she did not know whether or not her grandson wanted to get married or whether he had a thing for Rong Mo or not.
If he didn't and he later finds out that Rong Mo is a woman and that's the reason why Granny Shang wanted to get them together, he might have a stark reaction about it and keep his distance from Rong Mo.
For safety's sake, the current situation was the best for them to slowly build up their feelings!