XTG files are produced directly in QuarkXpress using the default CopyFlow export functionality. It doesn't seem any other program would be capable of QXP to XTG conversion.
Convert QuarkXpress project file to QuarkXpress tags file.
We found 2 software records in the database related to .qxp to .xtg file format conversion.
Microsoft Windows software - convert qxp to xtg on Windows
A design software used to create rich documents for web or print
Convert from qxp file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned qxp to xtg. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert qxp to xtg on OS X
Software fro document design and publishing