How did so many people get away with cheating during the written portion of the Chunin Exam?
I watched this in the anime recently and this is what I recall.  Most of this is actually explained throughout the exam or at the end.

First, the exam was set up that each person started off with a full 10/10 score one the test.  For each answer they got wrong they lost 1 point.  For each time they were caught cheating they lost 2 points.  Anyone caught cheating 5 times failed and he and his team were disqualified.

As Sasuke realized early on, the questions were so hard that no one would know all the answers and that cheating was actually encouraged (this is also confirmed by the fact that there were "students" planted in the room who had all the answers to cheat off of).  The test really wasn't one of knowing the answers (as the end of the exam proves), but rather one of how well they could gather information.  I don't believe it was expected that they would never get caught, but rather that if they were doing this on a real mission that they wouldn't be so sloppy as to get themselves killed.

So is it possible that someone noticed Gara or Tenten cheating?  Certainly.  Did they get caught enough to get thrown out? No.  And that's the key.