The name “Gerlach” isn’t as rare as I thought. Neither is it common. The distribution has been calculated by using data from German telephone books. If you want to know something about the distribution of your own family name in Germany go to
https://christoph.stoepel.net/Geogen.aspx.

‘Namensverteilung’ means ‘distribution of name’
‘weniger als’ means ‘less than’
‘Vorkommen’ in this context means ‘people counted’
‘bis’ means ‘up to’
‘mehr als’ means ‘more than’

 

Gerlach