Most probably you need to contact Navteq, because this is the main source of data for Garmin maps. But it could depend on a region or particular map, some of them are created by regional Garmin partners.
On navteq site you will find easy way to correct or supply missing data. But I can’t tell if or how fast your input will appear on Garmin maps.
Oh, one can download maps form Garmin. This way works all subscriptions to City Navigator maps