Hey!! You could go with the Sk-ll facial treatment essence. I don't know how much you wanna put on the gift but this essence is about 200$ haha. People RAVE about it and it's a really really good one in my opinion. Pretty sure you can find it at sephora :)
Btw its actually so nice of you to do this! Wish my boyfriend did the same hahaha
Hope i help you :)
Honestly, while I appreciate the sentiment and that you want to do something nice for your gf, I would be not super happy if someone picked out semi expensive skincare for me and it turned out to be something I couldn't use/didn't like or need. You might be better off giving her a "gift card" for a certain amount that you're willing to pay for what she wants and where she wants it from. Maybe you guys could sit down together online or go out to the shops together if there's something near her while you're visiting and pick stuff out.
Hm! Age (approximate to 5-10 years) and skin type (dry, normal, acne, dehydrated?) would help! Also whether she likes things that smell or stays away from fragrances like the plague, and whether she's ok with animal ingredients (collagen, snail mucin etc.). Depending on whether she's similar to me or not, I may be able to recommend some pricey but lovely stuff that's definitely not in the budget range.
While there are fancy, expensive brands like Sulwhasoo (perhaps most famous for their ginseng eye cream), in the AB world, more expensive doesn't translate to more effective often at all.
If you can provide her ingredient sensitivities (e.g. fatty alcohols, silicones, ferments, hyaluronic acid), I can give you some common products that people here cite as all time favorites. It'd be helpful if you had a product type that you were looking to buy too. Her skincare concerns would be good to know as well, of course.