"Paste all of the downloaded files into the gegl-.4 file" What downloaded files? And if we're getting them from the gegl-.4 file, what other file are we pasting them into?
Check if you have a layer mask in your layer or your check opacity; make sure that your opacity is full; try on a different layer or different type of text; if it still isn't working for you, then I think you should contact its developer; I have given a link in the description.
You can usually learn how to install it on Linux from its GitHub page(github.com/LinuxBeaver/GEGL-Effects---Layer-Effects-in-Gimp-using-GEGL/tree/main/GEGL_EFFECTS), but you can also visit the discord of Gimp, where you get the answer from its creator.