Looks real to me. The original Japanese games did use the standard grey cartrdges, so that's fine.
The label looks fine, colours all in proportion (nothing too bright or too dark).
And it looks like it says 'Nintendo GameBoy' on the cartridge, too, which fakes usually do not.
You very rarely see fakes made to look like the Japanese version, because more people get won over by the game being in 'never before released english'.
It looks perfectly real to me - not a fake at all.