A SFG file is a compressed project created by Synfig Studio, an open source application used to create professional 2D animations. It contains a .SIF animation file, but also includes the project's assets such as embedded images, audio, etc., and is compressed with Zip compression.
SFG files are one of three file formats used by Synfig Studio, along with .SIFZ and SIF. The SFG format was introduced with the release of Synfig Studio 1.0. The format is similar to the SIFZ format but is compressed with zip compression instead of gzip.