Just as all four men start fighting with one another, an obvious dummy dressed as Gilligan is thrown into the fray from the right hand side of the screen.
It seems unusual that Sam would still be alive if the gangsters he was with had pulled off their crime in April of 1906. That would have been 59 years from the time line of the castaways. For Sam to have learned to mimic the gangsters voices/words he would have to be old enough. Even if he was a young bird at the time, he would still be very old. Also, the container the crackers were in would've corroded by then, not to mention the condition of the crackers themselves.
Gilligan's nervous instinct to whistle through his nose in times of danger, never becomes an issue at any other time in the series.
When Sam the Parrot leads the men to the cave where the "jewels" are buried, the Skipper remarks that he has "never seen this cave before." However, earlier in the episode, the men crouched outside the very same cave to eavesdrop on the "gangsters" they thought were inside.