This is about https://github.com/marcmerlin/AnimatedGIFs (which recently added FatFS support)
Louis Beaudoin published a gif decoder for arduino, which was designed for SmartMatrix and Teensy, and used for the Aurora Project. It also required an sdcard since Teensy does not have onboard flash usable for an onboard filesystem (unlike ESP8266/ESP32).
With his encouragement, I took his code, and ported it to my FastLED::NeoMatrix library which allows using neopixels as matrices. I later added Smartmatrix::GFX Support to allow using the same GFX API to talk to both NeoMatrix and RGBPanel backends
I also added support for SPIFFS and FatFS on ESP8266 and ESP32 (built in flash memory filesystem on ESP chips, which allow working without any sdcard at all).
Here is the resulting code: https://github.com/marcmerlin/AnimatedGIFs
You can look at AnimatedGifs for the keys that are supported over serial port.
If "I just want to to display a GIF on my NeoMatrix", then I direct you to SimpleGifAnimViewer.ino which is a mere 60 lines of code if you remove the comments.
If you are ok including one more file, you can do it in 10 lines of code instead, see SimpleGifAnimViewer2
If you'd like to use the faster FatFS (over SPIFFS) there is a Makefile to generate img.ffat. See Using FatFS FFat on ESP32 Flash With Arduino for how to generate a FatFS image on your linux computer.
Here is an example video and pictures of NeoMatrix 32x32 and RGBPanel 64x64:
Here is a version running on a 64x64 RGBPanel backend with 2 different kinds of diffusers: