I solved this by writing a bash script that uses ImageMagick to turn all the GIF files in a directory into spritesheets (PNG strips). The tricky thing though is that my GIF files often dont have the same number of frames.
I'm gonna try to have my bash script discover the number of frames in the GIF before converting it to a PNG strip and then add that number to the end of the filename. ie. foobar-14.png. When using sprite_add i can hopefully read the filename, split it on "-" to grab the number of frames.
Something like this:
// foobar-14.png
var file_info= string_split(filename, "-");
var frames = file_info[1]; // 14
spr = sprite_add(working_directory + "STRIPS/"+fileName, frames, true, true, 0, 0);