This all depends on how you are compiling. I'll assume you are using Flash IDE to compile. Flash has a source path lookup mechanism. This can be accesed through the fla (under File->Publish Settings->Flash (tab)->ActionScript). If the file Scaffold lives at the root level of a class path then there is no need to import it. The default class path includes the directory the Flash file resides in and some Adobe directories. There is a mechanism for managing class packages ... I think it was the old Macromedia Extension Manager. It would copy classes into the default class paths when the package was installed - so that could be the case. I have no familiarity with Gaia itself.
So there are 3 options. First the file Scaffold.as is in the same directory as your fla. Second the file Scaffold.as is in a default Adobe include directory. Finally the file is in a directory referenced in the fla's class path.