Converting .json files to .jtk2d files
Mod the Gungeon API 1.5.5 can recognize .jtk2d files as sprite definition metadata files. That means just like .json files, they can define things like hand positions for guns. You can either rename your .jsons manually, or you can use the Json to Jtk2d tool (Windows only) to automatically convert all .json files in a folder.
To use the tool, just drag and drop the folder you want into it (usually it's the sprites folder) and it will automatically rename all .json files in that folder and its child folders. If you don't drag and drop, the tool will ask you to manually enter in the path to the folder. Be careful when using the tool though, as it doesn't look into the json files it renamed, meaning if a json file that isn't a definition metadata file exists, it will also be renamed. An example of a file like that is the manifest.json file required for thunderstore uploads.