The 28-year-old, who came fifth in the same event at Beijing 2008, set a new Olympic record of 691.9.
Gray also beat the Olympic record in qualifying with a score of 592 - just two short of a world best - and never looked like relinquishing her lead in the final.
Gray admitted she had prepared for the British weather, and that preparation certainly came in handy as the wind blew and rain fell during the qualifying round.
'It was definitely a little windy but we've done so much training around the entire USA,' she said.
'We trained in all kinds of conditions which really prepared me for this.
'In the test event I wasn't prepared for it but we really worked on it.
'And I needed patience. I was on top of it today, it was just awesome!'
I was on top of it today, it was just awesome!Jamie Lynn Gray
Maksimovic was thrilled to land silver, and said: 'I'm still in shock, I still don't know how big this is.
'I was crying because of happiness and because I did what I wanted to do.'