It looks like it should be possible to create ERDAS IMAGINE .gmd files from TIFF image sources. Or some sort of import and use of the image data is possible in creation of these graphical models. No specific information regarding this conversion could be found however.
Convert Aldus Tagged Image File Format (TIFF) bitmap image to ERDAS IMAGINE graphical model.
We found one software available in the database for this conversion.
Microsoft Windows software - convert tiff to gmd on Windows
A Geospatial data authoring software
Convert from tiff file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.