I have always wondered but I never knew how people do it
I think you just use PROP to edit the life values for the animation.
I've found it helps to sing softly to the goo
You do it seting the life of goo to a value.
In that value of frames the goo will disappear.
If you pause the game, you put a rectangle of goo, you press shift+1 (life view) and you draw a cat with prop (life 50) and then you press 8 (nothing view), you unpause and wait 50 frames you will see a hole that looks like the cat you draw some steps ago.