Germany wrapped up their Nations League campaign with an entertaining 3-3 draw with England at Wembley on Monday.
Gundogan was on the scoresheet, tucking home the opener from the penalty spot in the 52nd minute after Harry Maguire felled Jamal Musiala in the box.
Kai Havertz scored two more for the Germans but Luke Shaw, Mason Mount and Harry Kane hit back for the hosts.
Germany finished third in the group behind Italy and Hungary ahead of the Mundiale but Gundogan is confident they will perform creditably in Qatar.
"It's not unrealistic," the Manchester City midfielder said after the game.
"Of course, a lot has to come together. I don't see a team that is miles ahead.
"We have shown over long periods that we can do it at the highest level. We have to try to do that for 90 minutes. At a World Cup you have much less leeway to make mistakes than today."
Germany head coach Hansi Flick agreed with Gundogan's assertion that the team must cut out individual mistakes.
"The first half was balanced, in the second we deservedly led 2-0 – then we made individual mistakes," he said. "It must not happen that we give up a lead like that.
"But we came back, that's the positive. It was a good test, we take a lot of positive things with us, but also negative things. There's some work for us to do, but we're optimistic, otherwise we could stay at home."
Germany exited the last World Cup in Russia at the group stage.
