Below is a distribution map of GANDERs and GANDARs taken from the Index to the 1881 Census - a project led by the Church of the Latter Day Saints ('Mormons').
The map was kindly compiled by Geoff Riggs, a fellow member of the Guild of One-Name Studies.
As you can see by the blue for Sussex, this is the county that was - and still is - the most densely populated by GANDERs.
Looking at the green for Kent would perhaps suggest an even spread of the name, whereas the number of GANDERs living in Kent were very largely those in the Tonbridge/Tunbridge Wells District of Kent - which means those Kent GANDERs could be considered an 'overspill' as this District borders onto Sussex. There were very few GANDERs elsewhere in Kent. A perhaps more accurate picture can be gained by a look at the maps elsewhere where I've shown the distribution of Birth Registrations broken down by Registration Districts in England and Wales.
Also with the bright yellow for Essex. Just 13 people giving an Essex address and all these addresses were so close to London they are indeed no longer 'Essex' but now in Greater London.
One surprise for me was in finding 7 GANDERs in Banff, Scotland, giving Scottish birthplaces. I suspect they are 3 generations of a GANDER incomer who was deceased by 1881. One day I might find out. However, given the low density of the overall population of Banff compared to the southern counties of England, those 7 GANDERs warrant a 'green county'. It suggests there were more GANDERs in Banff than there were in Warwickshire (bright yellow) - which actually had all of 14 GANDERs!
Further distribution maps of the name - ranging over the period 1837-1910 can also be found in my Maps section, but these are for England and Wales only.