Yep... to expand a bit on that...
"Greenies" allow for two putts on each particular hole's green. A par 3 - "GIR" is "on in one"... a par 4 - GIR is on in 2... par 5's ya gotta be on in 3 strokes to be considered "on the green in regulation".
But remember... "on" means on the green , not on the fringe of the green. I'm pretty sure it's only tabulated during "ranked rounds" (not practice). *someone correct me if I'm wrong
The % part of it is just a calc of how often you're there as opposed to how often you're not... how often you get there with two putts to spare, out of how often you've tried to get there "in regulation". It's a good way of you knowing (and others knowing) whether you're a "balls to the walls" type of player, or one who's conservative enough to care about such statistical drivel. Nahhhhh... j/k....lol!
This stat can help here at WGT if you pay attention to it early enough in your "career", but doesn't mean much later on. For example, currently I probably hit 75% "GIR"... but because I used to not give a **** about it, and didn't care to monitor it, the exhorbitant number of my shots to the greens that I've taken presents a skewed percentage that's totally "out of whack" compared to what my "actual" GIR is now. It IS accurate, but not actual, just as alot of other "%" calc's are.
Conversely, alot of "early-on" high GIR players who've moved up tiers and have a high volume of approaches to the greens continue to have their GIR reported as "high", even though they may be struggling within their newer tier, making it onto the "greens in regulation" a lot less frequently, accurately reflecting their career GIR, but not their current tier's GIR.
It's something that longtime players here wish could/would be "reset" (at least when moving up tiers), so as to be a more valuable stat, but WGT hasn't deemed this change necessary, as yet.