What really matters is how intimate you two are and how long you've been together. If it's something more recent, 100 euros is already generous and shows you're paying attention. If you've been together longer, maybe you could think about something between 150 and 250 euros, depending on what you usually spend together on leisure stuff.
The thing is, a digital gift card from a store works better when it comes with something personal, you know? Like, just the gift card can feel a bit cold if all you're doing is transferring money. If you can pair it with a small physical gift or even a nice dinner, then it becomes something where she'll really feel the care you put in. I know people who just give a gift card and then end up regretting it because it doesn't end up being that special moment of unwrapping something.
In the end, if money's tight, 100 euros is totally acceptable and it's far from being stingy. But if you can stretch it to around 150 or 200, it depends on what you both earn and how much you want to show. What matters is that this feels comfortable for you and doesn't leave you strapped afterwards.