One would assume that its to do with the gender of the cat, with "el gato" reffering to a male cat, and "la gata" reffering to a female cat.
How would you know the difference between male and female, if they just say ´the cat´?!
If it tells you to translate into Spanish, it should and must accept "el gato" or "la gata". If it doesn't accept one, it should be reported. Both are correct.