Gruner is one of the most versatile white wines. Period. Styles vary from Austria to Italy (Alto Adige) to California. Last California gruner I tried was Alfaro and it was fuckign awful. Maybe they've gotten better since then. This was back in 2015 or so, so it was probably 2013-2014.
Age-worthiness depends heavily on ripeness and acidity. Gruner Smaragd from Austria (Wachau) or Italy (Alto Adige), if picked at the right time, should last 10+ years, even if they were not made with that intention.
Short answer: depends on who makes it. If the grapes were at phenolic ripeness at picking, and acidity is high enough (and pH low enough), and stored correctly, yes, a gruner at 5+ years should be wonderful. A little bit of skin contact helps, even just a couple hours.
What is the 2015 about which you speak?