We got the adorable couple -- and their cute 1-year-old baby, Gloria -- out in Bev Hills when RGIII made the revelation to TMZ Sports.
"Track is her realm," he says. "She owns the track."
Big-time praise for sure ... considering the NFL star once ran a 4.38 40-yard dash and did do some track events at Baylor.
But Grete's legit on the turf ... she crushed the heptathlon at Florida State -- once running 13.92 in the 100-meter hurdles -- and is making a serious run to compete for Estonia in the 2020 Games.
As for the real takeaway from all this ... watch out for baby Gloria in about 17 years!!