Bat Mitzvahs are a celebratory coming of age ritual in the Jewish community for young girls. If you have attended one you know this is more than just a birthday party but a symbolic celebration to honor the young girls elevated status within her religious community.
Gifting for this kind of event comes in various options which depends on the girl, your relationship to her and how well you know her. As suggested money is pretty standard but it is advised to give money in multiples of 18 so instead $100 you would give $108. The reason behind this tradition is because in Jewish numerology the number 18 translates to “Chai” meaning life, therefore you are blessing them with a long and successful life. The closer you are in relationship to the girl the more generous you would be with a financial gift.
Alternatives to monetary gifts revolve more around the specific girl and your relationship to her. If she were you granddaughter this might be a time to gift her a special piece of jewelry or a small family heirloom. The more distantly related or non-familial relationship with the girl the less intimate the gift could be. You can get away with a gift that reflects her hobbies and interests of girls her age.
The bottom line is that gifts should be about strengthening relationships between the gift and the recipient. When you can think of it in that light the right gift is one that makes the recipient feel valued and appreciated.