Of course, the natural tendency is to think Kobe rocks in this situation. But as in most things, what we believe based on a memory from the past is rarely the truth.

In the five year period, 14 players had made 10 or more game winning FG's at games end. Here are the 14 ranked by their shooting percentage in that situation:

1.  Carmelo Anthony
   
2.  Allen Iverson
   
3.  Ray Allen
   
4.  Rashard Lewis
   
5.  Ricky Davis
   
6.  Ben Gordon
   
7.  Paul Pierce
   
8.  Lebron James
   
9.  Dirk Nowitski
   
10. Vince Carter
    
11. Dwayne Wade
    
12. Joe Johnson
    
13. Jamal Crawford
    
14. Kobe Bryant

What the prosecutor wants the juror to testify about is something that happened in the courtroom at the previous trial, a gesture made by one participant.

1.  (a) As to what the prior juror perceived while seated in the courtroom, that would not be hearsay. It would be a person testifying as to their direct sensory perception of an event and, therefore, admissible as long as it was both relevant to (proving or disproving) an issue in the case and neither unduly prejudicial nor cumulative.

There may be (it varies from court system to court system) some limitations on what the juror may testify to - as a matter of competency of the proposed witness - because of what I'll call "confidentiality of the jury and its deliberations" considerations.  That is, suppose there is a rule which says "jurors may not testify as to the content of their deliberations other than in the context of a proceeding in which misconduct pertaining to the jury is in issue".  The question would then become (a) "is this event a part of deliberations?" and (b) "if (a) is answered 'yes', is this a misconduct-pertaining-to-the-jury proceeding?"  Only if (a) is yes and (b) is no would the juror be disqualified on competency-as-a-witness grounds.  The scope and nature of when and how a juror may be competent to be a witness is a matter of local law in the jurisdiction.

But

(b) how is what the juror perceived relevant to an issue in the case?

I'd also continue to ask how does this proposed testimony tend to prove a fact in issue?  It's not like the defendant is on trial for making a hand-slash across his throat at a witness during the prior trial, the implication being a threat to that witness.  The defendant is on trial for something that happened outside the courtroom, and the juror wasn't there for that.  How does what happened inside the prior trial tend to prove a fact in issue about something that happened outside the prior trial?  

It Doesn't.

2.  Interpretation of the alleged gesture.

This, on the other hand, seems to be hearsay to me, but we may not have to even go there.  What would necessarily be required is some interpretation of the gesture.  This is necessary in every instance when a witness' gesture is in issue.  Remember the old "see a guy walking by outside, then raising an umbrella" hypothetical?  This is somewhat different.  The juror would be saying "I saw a guy walking by outside and raise an umbrella, so I concluded it meant it was raining."  This is attackable on any number of fronts, but the point is that the witness is interpreting the gesturer's gesture.  

The question which keeps popping into my head is "is this testimony the best evidence?"  In other words, would admitting it violate the Best Evidence Rule and thus take the hearsay question out of the picture entirely?

What we have here is a proposed witness who is supposed to tell us not only what they saw a witness "say", but also what it meant.  The proposed witness' perception, understanding, recollection, relational ability and sincerity may be impeccable in all regards, but is such a conclusion (of what the alleged gesture meant) within their capacity to testify to?  I don't think so.  As other commenters have noted, a wave can mean any number of things, yet we are to believe that the alleged gesture had one, and only one meaning (conveniently, the inculpatory one).  I don't see it.

If you want to keep after the hearsay angle, ask:  "what exception to the hearsay rule would allow this testimony to come in?"  I can't think of one.  

3.  The alleged gesture is a confection of prosecutorial imagination, or, The Record of the prior trial tells us all that happened and all that didn't happen.

You note that the Record of the prior trial does not reveal the existence of the gesture.  I would argue "it didn't happen" and base that on two or three separate grounds.

First, IIRC, the general principle pertaining to mistrials as a result of a hung jury is that it is treated as though nothing in the way of a trial had happened.  The actual testimony of a witness - on the record and under oath - is treated like a deposition of a non-party witness in a civil context.  It can be used to cross-examine a witness at the retrial ("You said 'a' under oath before and 'not a' now:  which is true?") but would be limited in terms of substantive admissibility because of Confrontation issues (i.e., the prosecution can't just move the transcript of the prior trial in and rest.  But all we know to have happened was recorded.

So, if the Record does not show the gesture, I argue it didn't happen.  Going back to the "guy walking by outside, then raising an umbrella" hypothetical:  the issue is not what the event described meant but, rather, whether it occurred at all.

Second,  calling on a juror to contradict the record of the prior trial may violate the oath, or the instructions to the jurors.  There is a standard principle enunciated to the jurors which says that in their deliberations their memory of the testimony, demeanor, etc., rather than their notes, shall govern.  But that only applies to their deliberations.  When the Court gets a hold of the case, the Record governs.  There is also another which says that they should deliberate on what actually transpired and that no one juror should take over the deliberations (nor should the others allow it).

In calling just this one juror, the prosecution is violating both of these principles.  It is placing the recollection of one juror over the Record.  And, it is allowing one juror's recollection to override the recollections of all the others.  (I would be sure that the prosecution had gone out and interviewed all of the jurors to find out which, if any, had noted the gesture.  If they are only proffering one, that means to me that 11 said "no memory of it" or similar, otherwise they'd be trotting the whole panel into court.)  So, demand that all the prosecution interviews of the other jurors be produced, as an initial matter.