His double strike for Iceland this week sank World Cup semi-finalists Holland in a Euro 2016 qualifier in Reykjavik.
Monk said: "Gylfi has been under-rated and could play for any of the top sides. But he feels comfortable here, likes the way we play and we allow him to express himself.
"It's about signing good players to fit with our style and he matched that. That's why we pursued Gylfi throughout the summer and he hasn't let us down.
"What many don't see is how hard he works as well. He doesn't stop running and doesn't get enough credit for that.
"His work-rate is infectious. When the other guys see someone working extremely hard they know they have to put in a shift."
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