The Germany international feels the Madrid star has little reason to be optimistic but has stressed that BVB are not in the Champions League final yet
The Portugal international netted his side's only goal in Dortmund's 4-1 win at the Signal Iduna Park and has been in superb form throughout the competition, scoring 12 times.
Nevertheless, Gundogan believes the previous three encounters between BVB and Madrid give Ronaldo little reason for optimism.
"I have no doubt about it that Ronaldo will realise that it will be very difficult against us," the Germany international was quoted as saying by Kicker.
"We are a very unpleasant opponent. He will have experienced that in the previous three encounters this season."
Despite Dortmund's 4-1 victory in the first leg, Gundogan has stressed that the tie is not over yet.
"We are in a good position after the first leg. However, Real Madrid are always capable of winning 3-0 at home."
Tuesday's encounter at the Bernabeu is scheduled to kick off at 20:45CET.