I have already tried hiking with her and she has a hard time with the uphill.
No, she was not having a challenge with uphill. She was having a challenge with your hiking speed.
Do you understand what uphill hiking speed to maintain to make sure that your GF is able to move the whole day?
You can use your or her heart rate as an indicator or just a "talk test" (talking to her all day and making sure that she can comfortably talk without running out of breath on uphill portions of a hike).
