This will be yet another Payforit scam. You don't say which "service "your bogus subscription was for, so it is difficult to give specific advice. Although there is a charge for sending the STOP text, it will not be more than 12p. You can add this amount to the refund amount you request from the company. However there will also be a customer service number you can call using goodybag minutes. It is important to take action to stop the charges, as without this you will be charged £4.50 each week.
These companies use vulnerabilities of the 'Payforit' system to obtain consumers numbers and sign them up to bogus subscriptions. Simply closing an innocent looking popup can pass your number to these scammers.
GiffGaff appear to have a policy of not helping members affected by these scams, so you will have to take action yourself to stop the charges and obtain a refund.
'Payforit' subscriptions are not strictly premium texts, although they are shown as such on your GiffGaff usage statement. 'Payforit' charges are applied directly to your account by a 'third party' which has acquired your phone number. GiffGaff make no check that you have consented to these charges, and refuse to allow you to bar them.
You need to be aware that GiffGaff leak your phone number to third parties when you access the internet using mobile data, and this is probably how your number was acquired in order to make these charges. The system which does this is called 'Payforit'.
Comprehensive and independent information and advice on dealing with these scams is on the payforitsucks website. This website won't blame you for being scammed and provides constructive advice about forcing the scammers to refund. Specialist and legal support is also available through the site if you encounter difficulties. Google "payforitsucks" or follow the link to the faq in my signature. The website contains detailed advice on the steps you need to take to stop further charges, obtain a refund and protect yourself from these scams in the future.
I hope you manage to resolve this matter. These scams are a disgrace to the mobile communications industry and GiffGaff should be doing more to assist affected members and to protect members from these scams.