If any program is capable of somehow converting or saving .gph (graph) to .tvc (tables) would be one from which both of these formats originate, Lotus 1-2-3. Not the latest one, but the previous version for MS-DOS. You would have to use that version of the program if you hope to somehow perform GPH to TVC conversion.
Convert Lotus 1-2-3 graph to IBM Lotus 1-2-3 table file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert gph to tvc on Windows
Old spreadsheet program from IBM
Convert from gph 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.