Do I have to migrate the contacts one by one or is there a way to export to a file from google and import from the file in DecSync?
As far as I can see, google allows exporting as csv, and DecSync allows importing vcf/ics files, so I would have to convert the csv to one of those formats if that is possible.
Just add a new adress book and choose “import an existing collection”. If you give it permissions, you can then select the Google account already on your device, and import it directly.