Lists all GEM tag identifiers.

Values:

IGMDTAG_ID_GEM_FORMAT	GEM metadata format identifier.
IGMDTAG_ID_GEM_VERSION	Version valie.
IGMDTAG_ID_GEM_HEADERSIZE	Header size.
IGMDTAG_ID_GEM_PLANES	Color map ID.
IGMDTAG_ID_GEM_PATTERNLENGTH	Pattern length.
IGMDTAG_ID_GEM_WIDTH	Image width.
IGMDTAG_ID_GEM_HEIGHT	Image height.