hi and welcome to the forum
if you already have a sim account with giffgaff make sure you are logged in when purchasing your phone,then you will not be asked to buy a goodybag.
if you dont have a sim account then when you go to buy your phone,you will need to purchase a goodybag,this is only so you can give giffgaff a try.
you dont have to buy a goodybag every month (make sure you dont click recur goodybag when buying the goodybag with the phone)
you dont even need to activate the sim when you recieve it with your phone,just put it away in a drawer and put the network you are using sim in it.
you can use giffgaff as pay as yo go by topping up your account with credit (minimum £10) this will last until you run out of credit,where as a goodybag lasts for one month only and you lose any minutes/txt or data left in the goodybag.
so it,s entirely up to you,whether you want to buy a goodybag sometimes or stick to topping up your phone with pay as you go credit,
your loan for the phone is totally seperate from goodybag and top up credit
all the best