It would be nice to have an option like the -imacros option from cpp. (I can't see a way to achieve this behaviour with gpp)

To cite the cpp manual:

-imacros file
Exactly like -include, except that any output produced by
scanning file is thrown away. Macros it defines remain
defined. This allows you to acquire all the macros from a
header without also processing its declarations.
All files specified by -imacros are processed before all
files specified by -include.