Can I use this sign for gathering people and gathering objects?
Yes, the sign shown in the video is a great general sign for collecting or bringing things together, which works well for objects and general concepts. For large crowds of people, you might sometimes see variations, but this sweeping motion is widely understood.
Does it matter which hand is on top when they meet?
No, it doesn't matter which hand ends up slightly above or in front of the other when they meet in the middle. Just focus on the smooth, sweeping motion of bringing everything into a central pile.
How fast should I sign 'gather'?
The speed of the sign can reflect the situation. If you are hurriedly gathering your keys and wallet to catch a bus, sign it quickly. If people are slowly gathering for an event, a slower, more deliberate sweep is appropriate.