Right now, you cannot stream a movie from the GS preferences. You need to do by manual streaming.
First set up your streaming in Gallery Show to Manual
Then Choose
File > Stream > Flags > Enable Write on Gallery Show Cycle
Then choose
File > Stream > New Movie Steam
Name the movie and where you want to store it
Then Run Gallery Show
IMPORTANT: When you are finished you have to close the stream.
File > Stream > Close Open Stream
