A woman is always happy to get flowers from her beloved or admirer.
But it's so common when a man gifts the flowers in person.
I think if you want to make a surprise for you gf, it's better to gift flowers to her incognito.
Buy a bunch of flowers, come to her house, and leave them in some visible place. No doubt, flowers should contain also a nice card where your gf would read your warm and sweet words. Additionally you could attach balloons to the bunch!
So, returning to home, your gf will certainly get a flood of good emotions!