Should you need it, you can re-pack TAR archives to the previously used TAR.GZ format with some archive handling utility. Alternatively, you can just use a program that supports GZIP and decompress it, as TAR.GZ is nothing else but further compressed TAR archive.
Convert Unix standard Archive format, Tape Archive 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 tar to tar.gz on OS X
A program for creating Mac OS X disk images
Convert from tar 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.