Apparently you can convert .iso disk images do .tar.gz format with some disk image handling programs, like DropDMG. However, this iso to tar.gz conversion seems to be somewhat odd and perhaps not much useful.
Convert CD/DVD/HD DVD/Blu-ray disc ISO binary image to Old version Gzip compressed TAR Archive.
We have found one software suitable in our database for this conversion.
Apple macOS / Mac OS X software - convert iso to tar.gz on OS X
A program for creating Mac OS X disk images
Convert from iso 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.