What happens an order and offer are matched but there is a significant gap between them?
It depends on the exchange; they will have rules for how order matching is handled.

On at least one of the exchanges I have used, if there existed a buy order for 1 coin for $2000 and you added a sell order with a $1000 limit, then you would get $2000. If however, the buy order at $2000 was only for 0.6 coins then you would sell 0.6 coins at $2000 and the remainder of your sell order (0.4 coins) would be matched by the next highest buy order, eg $1950, provided that another buy order existed at or over $1000.

I would expect this behaviour to be more common amongst exchanges that what @chytrik described in his answer.