- What's difference between these two different types of chat rooms?
- Can a simple chat room be converted into a gallery chat room?
Your first question is simply answered by your second link (emphasis mine)
"Chat rooms are typically public and open to all. A gallery chat room allows anyone to enter, but only specific users may talk in the room, as determined by the room owner."
You can identify whether or not a chat room is a gallery chat room by a small lock icon in the upper left hand corner such as indicated below.
Moderators and chat room owners are able to change the chat room type.