not even close
This is from memory of a discussion about this long ago: The clips used in the intro have been positively identified. That you have not SEEN the scene you are looking for in the movies you've had suggested to you is for a good reason. The clip you are looking for- as originally described- is, IIRC, a CUT SCENE that DOES NOT exist on the video releases. And, yes, it was from either GALAXY OF TERROR or FORBIDDEN WORLD.
However the scene you later describe of a "big breasted woman" writhing around on a "gray floor" getting goo squirted on her is, if not from a porno, and really part of the montage in question, going to be from an 80s era ROGER CORMAN production. I'd guess DEATHSTALKER as there's a scene of a largish breasted woman covered in mud, which could be mis-remembered as goo. (And Corman LOVED to re-use that scene.)
My suggestion: Go to IMDB dot com and look over Roger Corman's filmography prior to NOT OF THIS EARTH. Just be certain of which one you saw.
If you can't find a movie that matches I think, "think" mind you, I may have a possible solve for you: You may be mis-remembering the cut-scenes from the intro to NOT OF THIS EARTH with the crazy cut-scenes used in ROGER CORMAN PRESENTS.
Ring any bells?
This was a cable series of DTV movies. In the opening montage there is a largish breasted woman getting covered in thick gobs of paint, which could be misremembered as goo. I suggest this a possible solve because there was as part of that series a NOT OF THIS EARTH re-remake which could be causing the confusion in your memory banks. Sadly I have no clue which of the ROGER CORMAN PRESENTS movies that scene was from but that may be where you want to start your search.
Hope this helps.