I am consistent with every SB that I am willing, even happy to help them with occasional gifts beyond negotiated allowance for things they need, but not so much for things they want.
For example: She may want a Dior handbag for $3,500. But what she needs is a $45 purse from Target. I use the same approach for holiday, birthday, and when appropriate Valentine's Day gifts.
This year, I am giving gifts to 2 SB's. One just moved into a new apartment - her first time living without a roommate. She got the place with zero furniture or even basic stuff like lamps, kitchen stuff, bedding, etc. and has been buying based on priority and budget. I just got her a work desk and office chair so she could do her work-from-home job without sitting on the floor or her bed. I am also getting her an Ember mug and a set of assorted teas. Another (former) SB (but we remain friends) is getting some much-needed new sneakers and socks.
One of the unintended benefits of my approach is that the gifts I give tend to hit big with my SB's. They see that, first, I was listening to them, and second, I really want to help improve their quality of life.
Gift cards a problematic, IMO. They may project a lack of care or thought about finding a gift with meaning. In addition, a gift card is often seen a poor substitute for cash. Why, because they will need to spend that card at the store which issued it (unless it's a Visa, MasterCard, or AMEX card). To illustrate my point, flip the script: What if she gave you a gift card for Kohls or Dennys? In the past I have seen SB's take the gift cards I gave them and flip them for cash, which ironically, they see as a hassle when I could have just sent funds to their bank account.
Last idea: Ask her what she really needs right now to make her life a little easier. Maybe ask her to list 3-5 things for you and you can pick one or two, assuming they are reasonable.
For the SB's who will not be getting a gift this year, I avoid scheduling them within 2-3 weeks before Christmas.
About budget: My general rule of thumb is 25% to 100% of her regular allowance.
Life is good
The Cat