Translation:The watch of Gabriella is older than that of Mary.
I see that you already got a very good answer to this question here (https://forum.duolingo.com/comment/26444389), but I'm including the link to it in case someone else comes across it and has the same question. It's strange that those two questions are identical, and yet they have different URLs. I came across this one during my lesson today.
"Gabriella's watch is older than Mary's" is a much more natural sounding English sentence than "The watch of Gabriella is older than that of Mary," and expresses the same grammatical concept.