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. |