Karetsas wowed the fans in Athens after rejecting Belgium to commit his future to Greece.
He put in an impressive display but couldn't breach Gordon's defence.
Now the keeper admits Steve Clarke's side will need to stop Karetsas' threat tomorrow.
Gordon said: “He looked fantastic when he came on. He's someone we'll need to look out for in the second leg.
“He's a great young player but it's about what we do on Sunday to try to nullify that threat and create chances of our own.
“I haven't seen many debuts like that from someone so young. It was an impressive first cap for him but thankfully we managed to keep him out.
“We'll see what Sunday brings and he might get more minutes at Hampden. But I'm sure we'll have a plan to deal with that as well.”
Karetsas can't wait for another crack at the Scots and is confident his side can turn the tie around.
He said: “We lost but played well in the second half and that is how we have to play on Sunday.
“If we play like that, we will win. We had many chances but we did not manage to put the ball in the back of the net.
“The atmosphere ...