GNU AWK it's literally copycat of Unix AWK having all of the functionality of the original AWK without being bound to the original source. So is GCC vs any vendored Unix 'cc', 'ld' and 'as', where GNU GAS was the alternative.
Again, there's GNU Bash against Unix SH, with the same exact flags for interoperativity. Ditto with Alpine against Pine, or GNU Nano against Pico with the literal same interface, commands and layout. And these are older than TTD itself.
Should I go in? Lesstif against Motif. If you installed Lesstif tons of Motif stuff would work straightly as is, as XPDF did. Another one? XMMS. Once you skinned both the were the same.