The Holland U21 international arrives from Heerenveen and Mowbray said: "We will lose Gianni to the Olympic Games but we knew that from the very start. He's been in the Dutch team right through the build-up.
"He won't be coming away on tour with us to Holland and Germany but then he was only playing for the Dutch U21 team three weeks ago so he will have a week off and then start preparing for Beijing."
Mowbray continued: "I first saw Gianni playing against us last year in a friendly for Heerenveen. I liked his attributes and I had the opportunity to go and watch him three or four times live.
"I tend to buy assets in footballers and, when the chance came, we made an enquiry and I'm delighted he's joined us.
"Now I hope he can adjust to the Premier League very quickly. He's the right age, has the right athleticism, the right qualities on the ball. I'm confident he'll be a good acquisition.
"It's a big step but, what I like about Gianni, is the confidence he carries around with him. I worked with a few Dutch players at Ipswich and they do have a certain swagger about them.
"If you manage that properly, you get the best out of them. At the same time, He's someone we've brought in for today but also for tomorrow.
"He will get stronger and stronger, and he will need to, because the English game is a lot different to where he's just come from."