The Dutchman spent two seasons as Erik ten Hag's No2 at United before leaving last summer.
Van der Gaag was named Zurich coach in May and says he treasures his time with United.
"I look back on my time at Manchester United positively," he told Givemesport. "I think it was two wonderful years. They were very positive and I learned a lot.
"The first year was very positive because we finished in third place, won the Carabao Cup and went to the FA Cup final.
"I think the only disappointment, in my opinion, was that we could have been better in the Europa League. But I think the first year was very positive, so everyone expected that we could improve in the second season."
Van der Gaag ended his time at United with an FA Cup winners' medal and added: "That was a very strange season (2023-24). We were not good enough in the Premier League or Champions League. There were very strange results as well because there were games we were winning comfortably and then drawing or losing.
"But, in the end, we won the FA Cup. It puts everything in a positive way, so that was important.
"They were two years when I learned a lot, especially about the way the Premier League functions, the way football is played and the tactical variations. It was something that I really enjoyed."