Marshall is currently with the Scotland squad but is third choice in manager George Burley's plans behind Allan McGregor and Craig Gordan.
However Gunn is adamant the 24-year-old will get the opportunity to add to his two caps if he remains patient and keeps performing at domestic level.
"It's frustrating because you feel you're playing well enough to be included," Gunn told the Norwich Evening News.
"But if you aren't in the starting line-up it's because you are around some top quality goalkeepers at the same time.
"In that situation you just have to be patient and hope for your chance, whether it comes in a friendly or a qualifying match. David has the qualities to be an international goalkeeper."