SDK folder contains documentation, examples and original quests to look info from. Main thing is to understand the relationship between mod/quest files.

- .mmxml file contains quest info and references for new xml data and bytecode file .bcd
- .gplproj file contains references to your gpl code files for majesty compiler (Gplbcc) to compile
- makegpl needs .gplproj to get references from it to turn your gpl to bytecode .bcd
- Basically quest/mod needs only mmxml and bcd to function. Also .xml if you add new content. All other files are optional.

some tips:
- don't create *.txt file and rename it to *.gpl. Instead copy existing .gpl and replace its insides
- avoid modifying original game functions. If possible use custom functions for compatibility with other people's mods
- don't multiply integers by decimals.
- run MakeGPL in your system's CLI to check for errors

If you just change values in .xml files make sure to include that .xml file into .mmxml. You can see examples of that in documentation or download some recent workshop quests/modes

In short, yes, you need to use that tool. At least to create mod which would contain modified .xml data.
https://prnt.sc/1z6h41y

Thanks Victory.