The default HTTP transport implementation for Go isn’t supported when running in AppEngine. Trying to use it will result in the following error:

http.DefaultTransport and http.DefaultClient are not available in App Engine. See https://cloud.google.com/appengine/docs/go/urlfetch/

To fix this, you need to use the http.Client implementation from AppEngine’s urlfetch package (imported from google.golang.org/appengine/urlfetch).

uc := urlfetch.Client(ctx)

options := []maps.ClientOption {
    maps.WithHTTPClient(uc),
    maps.WithAPIKey(GoogleApiKey),
}

c, err := maps.NewClient(options...)
if err != nil {
    panic(err)
}

nsr := &maps.NearbySearchRequest{
    Location: &maps.LatLng {
        Lat: latitude,
        Lng: longitude,
    },
    Radius: radius,
    OpenNow: true,
    RankBy: maps.RankByProminence,
    Type: maps.PlaceTypeRestaurant,
}

psr, err := c.NearbySearch(ctx, nsr)
if err != nil {
    panic(err)
}