According to the rules, you either get permission (like you say) or at least give the original author credit for using their resource, the latter in which is chief regardless.
Now, if the original author is still online and active, then of course you pay that user a visit for permission.
For all anyone knows, TerranUp16 has been gone for so many years, so it's likely the project is truly abandoned. I've seen threads where he says he's
given up on the GTW project.