All of these words describe a group of organisms, but each also has a different association that will change the meaning of your sentence if you use them.
A "flock" is a group of birds, especially a group of birds in flight. When used of people, it is used metaphorically, to say that the people are behaving, in some way, like birds.
When people "flock" to an exhibition, they are behaving like birds do when they travel in flocks; you often see birds arriving at one place and filling it quickly. When people "flock" to a museum, it means that people are arriving in large numbers.
A "swarm" is a group of insects, especially flying insects such as honeybees. When used to describe people, it means that the people are behaving, in some way, like insects. Where a flock of birds might arrive and depart quickly, a swarm of bees is in constant motion. For example, paparazzi might swarm around a celebrity.
A "throng" is a dense group or crowd of people. If people are "thronging" to a place, or a place is "thronged" with people, it implies that the place is crowded, that there is little room to move.
"Gang" and "platoon" are more specialized words, used to describe specialized groups of people. A "gang" is a group organized by internal ties. A crowd probably doesn't know each other; a gang does. A "platoon" is a specific subdivision of an army; a platoon is almost always made up of soldiers. These two can't be used interchangeably with "swarm" or "throng" or "swarm."
Here is an example of how you might use all of these words:
That morning, the townspeople flocked to the church for Melinda's wedding. Since early in the morning, Melinda had been surrounded by a swarm of hairdressers, dressmakers, and attendants. By noon, the crowd around the church was so thick that the limousine had trouble getting through the throng. Melinda and her gang of bridesmaids had to get out and walk through a narrow pathway that had been cleared by a platoon of soldiers.