I'm pretty sure you'll be allowed to use gcfsysd in your program, since he gives the source code along with it (hence the .c file). If you're gonna write your program in C/C++, you can also just stick the code in directly instead of accessing the .exe externally.
As for the LZSS algorithms, a quick search produced this page. It hasn't got any code straight-up, but it gives you algorithms for both encoding and decoding, which I'm assuming you can use across multiple languages.
As for the LZSS algorithms, a quick search produced this page. It hasn't got any code straight-up, but it gives you algorithms for both encoding and decoding, which I'm assuming you can use across multiple languages.