Hi there @ramsa1976
the giffgaff app uses an internet connection so yes, you will need to have some form of connection to the internet to top up your account using the giffgaff app/website. It doesn't have to be data, wifi will do.
I am assuming that you have an internet connection to post on this forum?
If you have no way of connecting to the internet then you can dial 43430 to top up too. I recommend using the website instead though