XnConvert utility supports even some of the very rare image format like the GRO, which is a HP-48/49 grob graphics image format. With that it should be quite possible to perform GRO to JPG conversion.
Convert HP-48/49 grob graphic image format to JPEG bitmap image format.
We have found one software suitable in our database for this conversion.
Multiplatform software - convert gro to jpg on any platform
Cross-platform batch image transcoder compatible with 500 picture image formats
Convert from gro 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.