GUTS is a set of musically inspired new TRAITS for FIST.
The PDF is 8 pages, with a clean, readable layout that mirrors and builds on the style of FIST core. It's got a lot of custom art pieces, all of which look rad and zine-y, and it establishes a lot of charm through visuals alone.
The TRAITS here are complex and weird, with lots of options for lateral play. A few are also a little on the strong side, so you may want to calibrate them slightly if they're being dropped into a more grim and desperate campaign. ELECTRO GHOST gives you extremely easy WAR DICE recovery, REVERSED lets you farm +CREATIVE off of teammates pretty easily, and GOD TOOL is a oneshot machine that you can just keep using.
The PDF is available in English and Spanish, an extra layer of work on the project that should be appreciated.
Overall, if you want to add some musical flavor to character creation (and get a bunch of really interesting and varied TRAITS in the bargain,) you should grab GUTS.