Gundogan lost a Champions League final for the second time in his career when Premier League winners City were beaten by Chelsea a fortnight ago, with Germany team-mate Kai Havertz scoring the winner for the Blues.
The defeat in Porto was the second time Gundogan, 30, has lost a Champions League final having also been on the losing side for former club Borussia Dortmund when they were beaten by Bayern Munich at Wembley in 2013.
"It's not easy to lose a game like that," Gundogan admitted Thursday at Germany's European Championship base in Bavaria.
"We (City) were working up to it for a year -- even eight years in my case. I lost the final again, so I'm extremely disappointed.
"Still, life goes on -- that's the beauty of competitive sport.
"Now I'm focusing on the next tasks and I look forward to the tournament (Euro 2020 finals) despite the defeat."
Gundogan was City's top scorer last season with 17 goals in all competitions.
He hit the net for his national side on Monday during a 7-1 thrashing of Latvia in Germany's final warm-up game before the Euro begins Friday.
Germany open their Group F campaign against world champions France on Tuesday in Munich, where they also host holders Portugal four days later.
"The anticipation is huge -- especially before tournaments like this. We can hardly wait," added Gundogan, who is expected to partner Toni Kroos in Germany's midfield against France.
He anticipates a busy night trying to stem the French attack, featuring Kylian Mbappe, Antoine Griezmann and Karim Benzema, and expects a thorough test of the shaky German defence, which has conceded 20 goals in 13 games this season.
"You know that you are playing against a team that has a lot of talented footballers. We may not have much possession," said Gundogan.
"It is all the more important that the guys at the back support us.
"It could be a key to the game, which could allow us to win a lot of ball and force the French to run after us."
Gundogan hopes his personal disappointment in Porto could even help Germany.
"Winning the Champions League gives you a boost and self-confidence, especially for Kai (Havertz), his goal will give him a lot of belief that the team could use," he said.
"I'm pleased for the boys who won (the final) -- they deserve it."