Yep. You have to make sure all the related data and graphics are in the right slot, too. You don't want to end up with Kimi in a Ferrari (yet). Or you could download a season pack (most since '88 are available somewhere if not on here) and load it into GP3Edit, export it to the game and off you go. Oh, don't forget to change which car you are driving. Could be confusing if you don't. The other thing to remember with GP3Edit is to save your carset to a new name for each set you want to use. Many of the tools like GP3Edit that have been created by enthusiasts don't check if you are overwriting an existing file so take care with them.
Other great tools for GP3 are:
GPxPatch which lets you put custom files into your game at startup
APE Advanced physics editor
Cmagic for installing Magic Data into track files
Notepad for editing text files (most of GP3 performance runs on text files)
PSP for editing bitmaps