Ok, this can be a problem with scratchbox. The headers you want are are in seperate folders given the package name. (i think pkg-config is meant to be called to get round this)

but anyway, here is an example to "fix" the gtk error. using a file manager, open:
/scratchbox/users/$USER/ and find the folder with the chinook_armel folder. go into usr/include inside the chinook_armel and go into gtk-2.0 and copy the folders inside there into the usr/include folder above it. it shouldn't be hard to figure out what else needs copying.