I bought the TurboGrafx version of Gate of Thunder a while ago. The disc contains Gate of Thunder and Bonk 1 & 2 as well. Unfortunately it's missing the back inlay. The seller told me that he had bought the game new and it was delivered without a back inlay. I wonder if that's true? Anyway is there a website for pc engine covers? I would like to download the cover for the US or the japanese release. I don't like cd cases without an inlay in my cd rack;-)
try www.cdcovers.cc or www.mega-search.net, im not sure at the moment if they have PCE covers or not, but they have covers for damn near everything else, so who knows?? could be worth a shot.
Gate of Thunder never came with a back inlay in the U.S., because the game was only available as a pack-in with the TurboDuo, or with the Super System card. Here's a thread where some folks made their own back inlays: http://www.pcenginefx.com/forums/index.php?topic=5325.0