Sterling’s arm is partly underneath the car and I wonder if his gun was on that side in his belt or pocket.
That seems to be so, at least it seems to be where the officers thought it was. His left arm was visibly pinned by at least one of the officers, and you can see his hand out and empty. But his right arm was what the officer on top of him struggled with for several seconds, and it seems like that after the 5-second mark, the officer on top also had his own gun drawn in his right hand.
Apparently around the 4-second mark is when one officer screamed about him having a gun, and at the ten-second mark the officer shot. So a lot happened in a very short period of time.