make new object obj_particle or something
set sprite as badly drawn 8x8 white blob

CREATE:

image_alpha = 0.6        //starting transparency
alphaMod = random_range(0.002,0.005)        //how much the particle fades over time
image_xscale = random_range(0.6,1.4)         //random scale
image_yscale = image_xscale
vspeed = -random_range(1,5)         //random speed
friction = random_range(0.005,0.01)      //how much the particle slows over time
amp = 0         //sine wave width
freq = vspeed * 20       //sine wave speed, larger is slower

STEP:

image_alpha -= alphaMod      //particle becomes more transparent
x = x + amp*sin(y/freq);        //sine function
amp = vspeed/random_range(1,6)
//path seems to randomly change in the original UT so this brings it closer to that

if image_alpha <= 0 {
    instance_destroy()      //bruh
}

OUTSIDE ROOM:

instance_destroy()


that's pretty much it, now you have to set up a spawner instance which will spawn the particles
between every 1 to 2 seconds, so random_range(1,2) * 60

tweak the rest till it's good