Generally speaking, the grammatical gender of a noun does not reflect the biological gender of the animal it refers to. So you can refer to a male cat as "ei katt", or a female cat as "en katt".
However, we actually have a separate word for female cats, so it's a tricky example:
en/ei katt = biologically gender neutral, no matter whether you choose the feminine or masculine form.
ei katte = a separate word that technically only refers to female cats. Not seen much in writing.