Some mods, e.g: ic2/thermal expansion have it so that, when you fill up their machines with a liquid, the GUI animates as if the liquid is glistening. Does anyone know how to set that up, an animated GUI? Thanks!

Well, you can do it either by using pegenerated images or using GL and its magic.

 

For pre-gen:

Make .png that has e.g 20 different (offset in time) parts representing some e.g progreass bar. Write an update in your GuiScreen#updateScreen that will update integer offset. Then in GuiScreen#drawScreen you can use that update-meter to choose different parts (offset in time) of that .png to render. That will create animation (kinda like gif).

 

For GL-based:

I am not gonna say anything since this would require more than basic understanding of not only GL but rendomly generated images (e.g bubbles). So if you need help here - be more specific.