The German speedster had already become the Gunners' second-youngest goalscorer in the Premier League earlier that season with a low driven effort in a 2-1 win over Swansea, and at one stage he was keeping both Lukas Podolski and Alex Oxlade-Chamberlain out of the team.
Arsene Wenger described him as a "great hope" for the future, and there was talk of Joachim Low taking him to the World Cup.
Fast forward two years and Gnabry is now 20, unable to force himself into Albion's first team.
In March 2014 a knee injury ruled him out for the remainder of the season and ruined any slim hopes of going to Brazil.
He spent the whole of last season on the sidelines just as Arsenal's midfield, buoyed by the arrival of Alexis Sanchez, evolved without him into a glittering array of world class talent.
Moving to the Black Country on loan in search of playing time, he's had 12 minutes of league football so far this season.
There are many Albion fans, and many Arsenal fans too, scratching their heads as to why such a precocious talent is being left to rot on The Hawthorns' bench.
Of course when any player goes out on loan there is no guarantee he will get into the starting line-up, especially considering Albion and Arsenal are in the same league and playing the same opposition.
But two weeks ago Gnabry curled a peach of a goal into the top corner as Germany's Under-21s smashed six goals past the Faroe Islands. And last week he had a hand in both goals when Albion's U21s beat Stoke 2-0 at The Hawthorns.
His admirers, and there are many in North London, are asking what more can he do?
To answer that question, you have to look at Tony Pulis' approach to academy football.
A critic of the U21 league, Pulis doesn't believe it prepares players enough for the step up into the cauldron of pressure that is the Premier League.
"Serge has come here to play games but he just hasn't been for me, at the moment, at that level to play the games," said Pulis.
"He's come from academy football and not played much league football.
"Does academy football really prepare players for league football? And we're talking about Premier League football here.
"As a manager you pick a team that's going to win a game of football.
"You pick your best team, you don't leave people out because you don't like them, because of this, that and the other."
Pulis is not a manager known for throwing the kids on to gain experience, preferring to play players he sees as reliable and opting to buy those who have racked up the years in the Premier League.
In his mind, proven is better than potential.
There is a truth in that, obviously, especially in defence where Albion's experienced centre-back pairing of Jonny Evans and Gareth McAuley have looked solid all season.
But when it comes to adding a threat going forward, sometimes the unknown is needed.
Gnabry has only played ten Premier League games in his career and none of those came in the last 18 months.
He has the pace to unsettle full-backs but he can't influence the game from the dugout, or worse still, the stands.
Since those 12 minutes against Chelsea, he's been an unused substitute four times and hasn't even been in the squad for the last three games.
Pulis is determined to get more from the wings, he brings it up at his press conferences every week.
Yet Gnabry is consigned to exile, and unfortunately for him, he doesn't look like returning anytime soon.
Callum McManaman has played just 21 minutes in the last five games and the 24-year-old from Merseyside is undoubtedly ahead of the German in the pecking order.
If he can't get on the pitch, there's almost no hope for Serge.