GIF or PNGs are Animations and no particles..
But you can use them as animation by using a PANEL with a window, or as a Sprite Entity with ent_animate.
Except of GIF.. you need to edit the GIFs into a line-stream animation, cause GS can´t handle Frame-Animations of a GIF ^^

for PARTICLES
Go to the 3DGS Manual, hit STRG+F and search for "PARTICLE"

And you´ll need this one: effect

You can take a look into the Wiki too. There, you´ll find this:
GS-Tools: Particle Editors
i prefer "Particle Effect Builder"