No, it is masculine but here "to" corresponds to "this" and "jest" was omitted.
audio is not playing... very good thing when you are doing the listening exercise..
It's what Duolingo app shows sometimes in puzzle exercises with Saxon Genitive. 's is a separate puzzle, so effectively you write "grandfather 's armchair". Don't worry about that.
"naszego" shouldn't really need to be translated. "This is grandfather's chair" implies "our".
"It is grandfather's armchair" was not accepted. Shouldn't that also be allowed?
It's different grammar and we are strict in keeping to it.
"This" in your sentence is a determiner: "This armchair".
"To" in our sentence is a dummy pronoun: "This is an armchair".
You may notice that "To" can only be a dummy pronoun here, because it wouldn't match the word "fotel" grammatically if we wanted to consider it a determiner. "to" is a neuter determiner and "fotel" is masculine.
Your sentence would technically be "Ten fotel jest naszego dziadka", but this sounds rather clumsy to me, I'd go with "Ten fotel należy do (belongs to) naszego dziadka".