It quite simple to reason out:
- Ripley submitted a report about the Alien.
- They lost contact with the colony.
- Every reason to believe the creature Ripley talked about was involved.
- Gorman is aware of this and tells Ripley they're trained to deal with such situations.
- Add to this the marines not giving a shit that Ripley once saw an Alien after they wake up; and then later don't give a shit during the briefing.
Not terribly difficult to come to the conclusion they've gone up against ET's before. And on top of that there's not a mention that this is first contact. Which one would assume to be a pretty big deal.