The Utrecht attacker spent time at both clubs in the early stages of his career, and expects a difficult encounter for the Eredivisie champions at the Signal Iduna Park
The Utrecht attacker, who came through the ranks of the Ajax academy and wore the BVB jersey in 2005-06, has admitted that it will not be an easy game for the Eredivisie champions, but could see them spring a surprise at the Signal Iduna Park.
"It will not be easy for Ajax, even though they've done fairly well this season. It's hard to say how the game will turn out, but there are always surprises," Van der Gun told Goal.com.
"I rate Dortmund a bit higher than Ajax, but they've definitely got a chance. They could very well cause an upset. I watched Dortmund's last game [vs. Bayer Leverkusen] and they were simply brilliant. They have a number of great players. They are very solid and that's why they have won two consecutive Bundesliga titles."
Van der Gun then went on to single out Jakub Blaszczykowski and Robert Lewandowski as Dortmund's key performers, but was he quick to stress that there's more to BVB than just the two Poland internationals.
"Blaszczykowski and Lewandowski are the standout performers, because they score a lot, but there's much more to Dortmund," he said.
"They might have sold Shinji Kagawa to Manchester United, but you don't win the German title if you have only two good players. The other players might not stand out as much, but they're reliable options and that makes Dortmund such a strong team."
The 33-year-old still knows a few Dortmund players from his spell at the Bundesliga champions, but is nonetheless hoping for a Dutch victory.
"I know a few of the Dortmund players who were still young during my team at the club. However, as a Dutchman, I hope that Ajax will come out victorious."