The 19-year-old made his bow for the Gunners against Stoke City in the Premier League back in 2013 - but the German admits he wasn't told he would start by Arsene Wenger until 20 minutes before kick-off.
The reason was that he stood in for the ill Theo Walcott - who was originally due to play the Potters.
But Wenger did not give Gnabry any hints beforehand, leaving it very late to tell you youngster he was about to make his Arsenal debut.
"I got told 20 minutes before kick-off that I was playing against Stoke City," Gnabry told Arsenal's official website.
"Before I had obviously trained with the first team and played a couple of friendlies but it’s not like a competitive game, so when I got to play it was very physical which I wasn’t used to but it was a lot of fun.
"In the first team we have players who have played in World Cup finals and they have a lot of experience that as young players, we don’t have yet. I think under-21s level is still a good level to play at and you can get a lot out of it but there is a massive difference in tempo, physicality and other stuff."