Gay and Gatlin are both 34-year-old Team USA sprinters, both public about their faith and both going for gold. They have also served suspensions for doping, and Gay was stripped of the gold medal he won in the 100-meter dash at the 2012 London Games.
Gatlin won a gold medal in the same event in Athens in 2004 but was suspended from 2006 to 2010. He previously served a one-year ban in 2001 after a test turned up amphetamines in his system.
The issue came to the fore over the weekend when Team USA swimmer Lilly King, who took the gold medal in the 100-meter breaststroke, said she believes in a lifetime ban on all dopers.
Gatlin's response: "I never even heard of Lilly King."
Yet when Gatlin entered the arena for Sunday night's 100-meter race -- in which he took a silver to Jamaica's Usain Bolt's third gold in the event -- there was more than a smattering of boos from the crowd.
Gay and Gatlin have tested clean so far in Rio. And Gatlin has spoken publicly about God's role in his return to the sport's good graces.
“I believe in my talent to the fullest," Gatlin said in court testimony related to his suspension. "And I think God is trying to be, my way of showing everyone that I can do this, I can run great times without even trying to use performance-enhancing drugs.”
Gay, in 2007, told a reporter: "I'm a religious man, so I really believe in my God-given ability, that I can do the unexpected. I really do believe I can break a record, or come close to it, or win a medal."
Both men have another chance to prove themselves. Gatlin runs in the 200 meter on Tuesday (Aug. 16) and the 4 x 100 meter on Thursday; Gay will run in the 4 x 100 meter on Thursday.