They put a temporary hold on the card (in case if you decide to keep a game longer), this link should answer the question you just asked. I would simply cancel a day or two before the trial runs out. If you have used the same address in the past for a gamefly trial, they will know. So as long as you are at a new address you should be fine, and using a new account.
And yes if your sign up is successful and doesn't disappear in my system, then yes you basically get to enter for free. It shouldn't cost anything as no one has said anything otherwise.
https://gamefly.custhelp.com/app/answers/detail/a_id/12/kw/credit
Why was I charged for a free trial?
When you sign up for a free trial with us, we perform a temporary authorization against the
credit card you provided upon registration. We do not receive
any funds from the authorization. The temporary authorization could take 3 to 5 business days to clear depending on your financial institution.
We require a valid
credit card for all GameFly memberships including free trials to ensure that if you choose to stay a member after the free trial or if you choose to keep a game, you are ready to go.
If you intended to sign up for a free trial but selected a prepaid instead, please
Contact Us and we'll be more than happy to assist you.