Because gata is a female cat. If a cat is mentioned without it's gender, as it was in this instance, it is automatically considered to be male. This applies to most animals.
On the other hand, I was just doing this review again and it did accept nossa gata. Perhaps someone reported it. Tehnically, it is still correct.
tuas means that multiple feminine things belong to
tu (you: singular/informal).
gatos is "cats", plural/masculine.
You were asked to translate "our cat", a singular cat who belongs to us (
nos). Therefore the only two possible answers are
nosso gato (our cat, who is male) and
nossa gata (our cat, who is female).
Because there must be agreement. If you say gata, you have to say nossa. If you say gato, you have to say nosso.
I wrote 'nosso gato', it said wrong. Apparently, according to the correction, I should have written, 'vosso gato'. So next time I wrote 'vosso gato', guess what? It was wrong, should have been, 'nosso gato'... What on earth happened there?
That sounds like a glitch. "Nosso/nossos/nossa/nossas" is "our/ours" and "vosso/vossos/vossa/vossas" is "your/yours (pl)".