I am not an expert on gyoza in Narita, but I googled and found there is a shop of "Gyoza no Osho" , a casual Chinese place near the Keisei Narita station.

Here are some pics of the gyoza served there.

"Gyoza no Osho" is a big chain of Chinese food. They have hundreds of shops around Japan. I don't call it very best but their food is above average. In most of the shops you can see them cooking in the kitchen from your table or the counter. So this might be interesting to western tourists.