Is the championship event real in America

This was filmed during the 89 season of the AVP (or, at least, part of it).

In the late 80's through a lot of the 90's, most of the events were Miller Lite events which paid out between ten and thirty thousand dollars. But Cuervo Gold Crown Tequila sponsored three events (The Triple Crown) which were dispersed throughout the season. Usually, they were in this order...

Clearwater, FL early in the season
Boulder, CO in the middle of the season
San Diego, CA at the end of the season and sometimes it was the last event

These events paid out huge purses; especially, for beach volleyball and they were worth a lot of (maybe, the most) points towards determining which team would be the number 1 team at the end of the season. Your goal was to, definitely, win all three of these for the money and the points.

In the movie, they call the end-of-the-season event the Gold Crown Classic. I assuming this is a quasi-reference to the San Diego event at the end of the 1989 season. So, in a way, yes, the event is a real event. In 1989, Barnes and Clark would have won a $50,000 check by the end of the movie. Once the sport really took off in the 90's, the top prize at these events was $100,000. There were a couple of years where Karch and his partner, at that time, Kent Steffes, won all three of these events in addition to a lot of the other Miller Lite events.