Trying to extract from .gpk file?

[podi07]

Hi there. I'm trying to get sprites from Touhou Sky Arena Matsuri Climax. From what I've managed to find so far, it should be in one of the .gpk files. However I've got no idea how to unpack these files. I haven't been able to find anything online, any help is greatly appreciated!

(I'm completely new to this and don't know where to start; I apologise if it's a dumb question!)