yes, you can use a GTC but i think you have to do it where you upgrade your trial WITH an activation code (which comes off the GTC). i've heard about this recently on forums and some people said they have to petition to get this resolved, so you may have to do that too if there isnt any automated way to do the activation thing on the site.
however, if you want to get a longer trial (21 day trial) you can ask me - just pm me your email.
p.s. yes, im worse than a car salesman by offering you a 21 day trial