Brendan Rodgers' side drew 1-1 with AEK in the first leg at Parkhead, but Gordon remains positive about Celtic's chances of progressing.
Celtic were also defeated 1-0 by Hearts on Saturday, which has led to murmurs of 'crisis' across Scotland.
Speaking to 'Celtic TV', Gordon said: "It's a massive opportunity for us to go and put in a good performance and try to get into the next round.
"We are slightly behind in the tie, so we have to go there and be very positive, and we definitely have the players who are capable and a team that is capable of going there and winning the game.
"That's what we need to concentrate on now. All the focus is on trying to get through the next round.
"This is what we have to go through at this stage of the season, and we still have to win games."