This is not a feminine name at all; it isn't even related to the name most contemporary people would think of, being Isabelle. Which is why it has a feminine reputation in the first place.
At any rate, if you name your daughter this, you're doling a cruel and unusual punishment. In France and especially French Canada this is a name frequently used for boys, and a female with this name would garner more than a few confused looks.
If you want to use this for a girl, I would go with Isadora. (Which I also personally really like!)
While I really like this name, I also am not fond of the meaning "Gift of Isis." Firstly, while I do like a person being considered a gift, I'm not so hot on the Egyptian goddess thing. Secondly, since it is talking about goddess, the meaning doesn't sound too masculine. However, and luckily, there are two Saint Isadores (may also be spelled Isidore), that help make up for the maybe not so desirable meaning.
Saint Isidore the Farmer: Born in Madrid Spain in the 12th century. It is said that angels helped him work the fields. Patron Saint of Farmers
Saint Isidore of Seville: A bishop of Seville in the early 600s. Wrote and encyclepedia of knowledge known as the Etymologies that was popular for 9 centuries. Helped establish a seminary in every diocese in Spain where he made sure they learned every branch of knowledge including art and medicine. Doctor of the Church.