what is Gonzalo ranting about

Basically Antonio and Sebastian are whining about their plight. They tease Gonzalo about his positive attitude. Gonzalo gives an emphatic speech about how lucky they are to even be alive,

"Beseech you, sir, be merry. You have cause / So have we all, of joy; for our escape is much beyond our loss." Gonzalo feels that this wreck is their life-changing destiny.